Monster Building Walkthrough: The Battle Mage

Welcome back to Pact of the Tome! Previously on Monster Mondays, we've covered the processes of creating a D&D 5E monster from scratch and upgrading an existing monster. This week, we're going to cover the process of building a monster that casts spells. In this case, we'll build a "generic" lower-level spellcasting opponent, suitable to bolster the ranks of any self-respecting humanoid encounter.
To simplify things, I'm only going to reference classes and spells from the Player's Basic Rules (also available as a webpage here), but you can look up their effects in the Player's Handbook if you want. I'll also reference the Mage creature from the DM's Basic Rules (p. 55) and Monster Manual (p. 347). To follow along with the process, you'll also want to pull out your Dungeon Master's Guide and turn to page 274. We'll be referencing the "Monster Statistics by Challenge Rating" table as well as the 20-step process for building a monster

Why restrict archetypes like
this one solely to players?
Artist: Unknown
(D&D 5E Player's Handbook)

Looking at the NPCs available in the Basic Rules and Monster Manual, you might notice that there's a dearth of arcane spell-casting foes at low-to-medium levels. While the Acolyte, Cult Fanatic, Druid, and Priest all cover divine spellcasting, the lowest-CR arcane caster (the Mage) is CR 6. We can do better than that! Certainly there's a place for the mercenary wizard that supplements a bandit gang, or the elvish spellweavers that accompany their warriors into combat. Let's build a Battle Mage.


Though we could build the Battle Mage as essentially a player character, let's try to do it using the monster creation rules. Why? Well, one big issue is that player characters pick up lots of incidental abilities that simply aren't relevant. For example, the Cleric's abilities to Turn Undead or a Wizard's Arcane Recovery will hardly come up in the context of a "monster" that is encountered for a single three-round combat. We will, however, reference the Wizard class for the Battle Mage's spells and spellcasting progression.
If we wanted to build the Battle Mage as a PC Wizard, we'd still be able to use the DMG rules to calculate its Challenge Rating. Keep an eye out for a later Monster Monday article that covers this topic.

Let's get started! We're using the 20-step process for building a monster from the Dungeon Master's Guide, so feel free to follow along with your own DMG.

1. Name. We've got one - Battle Mage.
2. Size. Although NPCs can be any race, they're mostly built for creatures about the size of humans. The Battle Mage is Medium.
3. Type. The Battle Mage is a humanoid, but could be any race.
4. Alignment. The Battle Mage, like a player character Wizard, could be of any alignment.
5. Ability Scores and Modifiers. Here's where it gets tricky. Let's build the stats for a Battle Mage essentially as those of a player character Wizard. Strength seems reasonable at 8, but for survivability this monster ought to have good Dexterity and Constitution scores - let's say 14 each. Intelligence clearly ought to be high, so let's say 16. Wisdom and Charisma are less important, so we'll set them at 12 and 10 respectively. (Coincidentally, these statistics are compliant with a PC "point buy" character with a +1 bonus to Dexterity and Intelligence.)
6. Expected Challenge Rating. The abilities of a powerful spellcaster can vary, so it's hard to be sure what the CR will turn out to be. Luckily, this step is mostly used for calculating proficiency bonus, which stays the same (+2) between CR 0 and 4 - we've got some wiggle room. Let's aim for a CR of 2 to start, which allows some space for this monster to be used in an encounter with other monsters against a low-mid level party.

At this point, it's important to take a break and figure out exactly what spells our Battle Mage will use. For a spellcasting monster, spells can dramatically modify Challenge Rating, whether it's mage armor or shield of faith boosting Armor Class, fireball providing damage output, or even levitate providing a way to avoid damage,
For simplicity's sake, let's give our Battle Mage the spellcasting abilities of a fifth-level Wizard. This means the monster will have access to 4 first level, 3 second level, and 2 third level spell slots. Additionally, with an Intelligence of 16, it'll be able to memorize eight spells total, and have access to four cantrips to cast at-will.
Formatted like it would be in an actual monster stat block, the Battle Mage's spellcasting looks like this.

Spellcasting. The battle mage is a 5th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +5 to hit with spell attacks). The battle mage has the following wizard spells prepared:
Cantrips (at will): acid splash, fire bolt, light, prestidigitation
1st level (4 slots): mage armor, magic missile, shield
2nd level (3 slots): blur, levitate, misty step
3rd level (2 slots): counterspell, fireball

Why did I choose these spells? Well, I first picked fireball, mage armor, and shield as the Battle Mage's primary combat spells. Fireball is the classic area-of-effect damage spell, mage armor provides decent defenses, and shield is a great reaction option to really boost armor class.

Magic Missile: the number one combat spell for
Wizards everywhere since 1974. Now available
in levels from 1 to 9. See your local Player's
Handbook for details or visit dnd.wizards.com
and search for "Player's Basic Rules."
Artist: Randy Gallegos

The rest of the spells fill in "holes" in the Battle Mage's fighting style and give it more options. Misty step takes advantage of the Battle Mage's bonus action to get away from an angry Fighter and behind some covering terrain, and provides a good use for second-level spell slots. Blur, counterspell, levitate, and magic missile each grant the Battle Mage options in combat and help it make better use of its action economy. If the Battle Mage has time to "buff" itself before a fight, it can cast blur to keep itself safe from enemy attacks. If the enemy forces appear to be composed mostly of melee fighters, it can cast levitate to avoid being attacked. Counterspell gives the Battle Mage a potent way to counter enemy spellcasters at the expense of its reaction (it can't cast shield) and the loss of a potential fireball. Finally, magic missile can fit into a first or second level spell slot after fireball runs out and gives the Battle Mage a way to combat foes with resistance to fire or high Dexterity saving throws.
The cantrips don't accomplish much as most 5E monsters are expected to last about three rounds. Even casting at maximum capacity, the Battle Mage isn't likely to run through eight spells in that time. However, acid splash (for multiple enemies) and fire bolt (for a single target) give it a way to cast effective offensive spells in a drawn-out battle. Light and prestidigitation are mostly for flavor.
Note that if we wanted to keep the Battle Mage exceptionally simple in play, we could cut the spells down to just a few - fireball, mage armor, magic missile, and shield. Since Magic Missile can be cast in higher-level spell slots, it'll fill in for most of the other options.

Now, back to the actual monster building process.

7. Armor Class. It's important that I took a break to choose spells, because calculating many of the Battle Mage's statistics rely on them to be accurate. Though a normal humanoid creature without armor (which mages don't normally wear) has an armor class of 10 + Dexterity modifier, there are two spells that modify this.
   The mage armor spell grants an AC of 13 + Dexterity modifier, and it's reasonable to assume the Battle Mage would have already cast it in most combat situations because it only requires a level 1 spell slot to cast and lasts 8 hours. We will consider the shield spell later in step 16 (Final Challenge Rating), but suffice it to say for now that each casting of shield (as a reaction) effectively adds 5 to the Battle Mage's AC for that round.
8. Hit Points. This is where the Battle Mage is really going to hurt. Even though Medium creatures built with the monster creation rules have d8 hit dice (as opposed to the d6 hit dice of player character Wizards), we're approximating a fifth-level character, so the Battle Mage will only have five hit dice, giving it 32 (5d8+10) hit points. Ouch! Consulting the table on page 274 of the DMG, this corresponds to a CR 1/8 creature - not the level of power we're looking for. We'll leave it here for now, however, as it's possible the Battle Mage's other abilities may compensate for this.
9. Damage Resistances, Vulnerabilities, and Immunities. None of the spells we've chosen for the Battle Mage grant damage resistance, vulnerability, or immunity, so we can essentially skip this step.
10. Attack Bonuses. Based on a proficiency bonus of +2 and an Intelligence of +3, the Battle Mage has +5 with spell attacks. If it ever finds itself making a weapon attack (for example, with a backup dagger), it'll probably use Dexterity and thus attack at +4.
11. Damage. Since we already know what "attacks" our monster is using, we can calculate this relatively simply. Remember, damage is considered in the "best case scenario" - that is, the monster uses its most effective attacks over three rounds and hits with all of them. We're assuming the Battle Mage casts two fireballs and then follows up with a level two magic missile, so we'll calculate damage based on that.
    Unfortunately, both of those attack modes present a somewhat thorny problem when calculating Offensive CR. Fireball could include any number of targets in its 20-foot radius sphere, while magic missile automatically hits on every attack.
    Though the DMG doesn't include any guidance on resolving these issues,  we can "reverse-engineer" the system to cover them. Looking at the table of Monster Features on page 280, we see that the Breath Weapon ability's damage is calculated assuming the breath weapon hits two targets, and each fails its saving throw. Against the typical 3-5 character party, this sounds about right for the fireball spell (assuming the Battle Mage doesn't want to hit itself or its allies).
    The magic missile spell's difficulties require some fudging, because calculating average damage output assumes that attacks hit all the time, even though they don't. An attack that does hit all the time, then, ought to be counted as dealing more damage. To avoid drowning in oceans of numbers, we'll roughly estimate a first-level character's AC as averaging 15 and note that CR 0-1 monsters hit at +3 on average. We can therefore surmise that standard monster attacks in 5E hit around 45% of the time - eyeballed at 50%. Since Magic Missile hits all the time, we'll double its damage to account for this.
    Finally, we calculate overall damage output. Fireball's 8d6 averages 28 damage, which doubles to 56 since it hits two targets. A second-level magic missile produces four 1d4+1 darts, and since we double its damage this totals to 28. Therefore, the Battle Mage's three-round damage output is (56 + 56 + 28)/3 = 47. Whoa! That's in line for an Offensive CR of 7. We'll keep it for now, though.
12. Save DCs. This is easy - in fact, we've already calculated that the Battle Mage's Spell Save DC is 13. Sounds pretty good!
13. Special Traits, Actions, and Reactions. The Battle Mage's only special abilities are its spells, and we've already accounted for those. We could add something else, but frankly, this monster is overloaded with options already!
14. Speed. Since the Battle Mage is a humanoid, we'll give it a walking speed of 30 feet and no other abilities.
    One thing to note here is that a flying monster of CR 10 or lower gains a +2 bonus to AC (for the purposes of calculating CR) if it has an effective ranged attack. Since the Battle Mage can cast levitate, should we give it this bonus? The thing to consider here is that levitate is a concentration spell that takes the Battle Mage a round to cast. That means it'll fall if its concentration is disrupted (taking additional damage), and if it doesn't have time to prep for a fight it will have to give up a round of casting fireball spells to rise into the air. With these mitigating factors, it doesn't seem like a good idea to grant the bonus.
15. Saving Throw Bonuses. Since the Battle Mage is modeled after the Wizard class, let's give it that class's saving throw proficiencies - Intelligence and Wisdom. Since this covers only two saving throws, it doesn't affect CR.
16. Final Challenge Rating. It's time to see how powerful our monster is!
    We'll calculate Defensive Challenge Rating by finding the Battle Mage's hit points - 32 - which correspond to a Defensive CR of 1/8. We then find Armor Class to modify this. The Battle Mage's AC with mage armor is 15, but we've got to consider the shield spell as well. If it casts this spell every round as a reaction (we can extrapolate that the assumed length of combat is three rounds from the damage guidelines, and the Battle Mage has three level 1 spell slots), it effectively has an AC of 20! Since every two steps above the CR 1/8 Armor Class of 13 moves the Battle Mage up 1 in Defensive CR, it ends up with a Defensive CR of 1 - not quite as bad.
    Offensive CR is easy to calculate, since we assumed two fireball spells followed by a second-level magic missile. This corresponds to an Offensive CR of 7, but we note that the Battle Mage's damage relies on its saving throw DC, which is 2 lower than the corresponding offensive CR DC of 15. The Battle Mage has an offensive challenge rating of 6.
    Averaging offensive and Defensive CRs, we find that the Battle Mage ends up with a total challenge rating of 3.5, which rounds up to 4. Higher than we expected, but still totally usable!
17. Skill Bonuses. There's really no need to give the Battle Mage proficiency in anything, but for flavor's sake we'll grant it proficiency in History and Arcana.
18. Condition Immunities. As a standard humanoid, the Battle Mage has no condition immunities.
19. Senses. The Battle Mage, as a generic humanoid, has no special senses. It does have a Passive Perception of 11 (based on its Wisdom score).
20. Languages. Since the Battle Mage is a generic, but smart humanoid, we'll give it access to any two languages of its choice.

And we're done! The final monster stat block looks like this:

Battle Mage

Medium humanoid (any race), any alignment

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Armor Class 12 (15 with mage armor)

Hit Points 32 (5d8+10)

Speed 30 ft.

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Str 8 (-1) Dex 14 (+2) Con 14 (+2) Int 16 (+3) Wis 12 (+1) Cha 10 (+0)

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Saving Throws Int +5, Wis +3

Skills Arcana +5, History +5

Senses passive Perception 11

Languages any two languages
Challenge 4 (1,100 XP)

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Spellcasting. The battle mage is a 5th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +5 to hit with spell attacks). The battle mage has the following wizard spells prepared:
Cantrips (at will): acid splash, fire bolt, light, prestidigitation
1st level (4 slots): mage armor, magic missile, shield
2nd level (3 slots): blur, levitate, misty step
3rd level (2 slots): counterspell, fireball
------------------

Dagger. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 5 (1d4+2) piercing damage.

Crazy that we went to all that work for something that more-or-less exactly resembles the Mage statblock, right?

It's a bit high-level for the Battle Mage, but
if you really want to terrify your PCs, cast
cloudkill on them. 5d8 asphyxiation
damage every turn adds up fast.
Artist: Claudio Pozas

Luckily, now we've got a great understanding of how the Battle Mage works. Ideally, it'll cast fireball twice for massive damage, burning through spell slots to shield every round and protect itself from damage. If it has prep time, it'll cast blur or levitate to increase its survivability - and since most of its damage comes from the fireball spell, spending a round to cast those might be a good idea even in the heat of battle. Misty Step is problematic because even as a bonus action spell, it precludes casting anything but a cantrip in the same round. However, should the Battle Mage desperately need a way out of melee, it can misty step to a safer spot, then drop a cantrip for some damage that round. Counterspell should be held in reserve (it competes for fireball slots), but if the Battle Mage can save its team from a nasty spell like slow or web, then it might want to go ahead and blow the counterspell.

I'll conclude the post with a few options for modifying the Battle Mage.

The first thing that comes to mind is changing the spell list - after all, can't any decent fifth-level caster fly? Sure, and you should feel free to add that spell, perhaps in place of levitate. Although you might be tempted to modify Defensive CR because of fly's added maneuverability, remember that most of the Battle Mage's damage comes from fireball, which fly competes for a spell slot with.

Other easy swaps might include lightning bolt instead of fireball (they're functionally identical, just with different tactical considerations), dispel magic instead of counterspell (both function to shut down enemy spellcasters), or silent image/suggestion as a replacement for a support spell (won't generally deal much damage, but increases tactical options).

For ease of play, you may actually want to reduce the Battle Mage's options - eight spells and four cantrips can be overwhelming to run in a tough battle. In this case, simply take away all of the Battle Mage's spells except mage armor (simply sets AC to 15), shield (+5 to AC until end of turn as a reaction), magic missile at second level (fires four 3 (1d4+1) force damage bolts that automatically hit), and fireball (deals 28 (8d6) fire damage in a 20' radius sphere; save DC of 13 for half). Since a combat will rarely take more than 3-4 rounds, assume each spell except fireball (2/day) can be cast an unlimited number of times. Simple and easy, and it doesn't change the Battle Mage's CR at all.

Finally, note that as a "generic" NPC, the Battle Mage can easily be modified to fit just about any race. Need an Hobgoblin war caster? Give the Battle Mage the Martial Advantage trait from the basic Hobgoblin, 12 Strength, and a longsword attack at +3 for 5 (1d8+1) slashing damage (doesn't modify CR, because it's not the creature's primary attack method). How about an elvish spellweaver? Darkvision, Fey Ancestry, and proficiency in Perception and you're good to go.

Today, we covered the process of building a spellcasting monster from start to finish, then went over a few different ways to use it in your campaign. Next week, we'll discuss modifying regular monsters like the kobold to give them some tactical tricks!