Monster Building Walkthrough: Swarm of Books

Hi folks!

This blog could use some content. My New Year's Resolution this year is to write a blog post at least once a week about some aspect of D&D.
The Swarm of Books is a monster I created for my Library of Bael Turath game. In one room of the library, the PCs found rows of bookshelves, all in the (mostly) unreadable Infernal language. They also found three pedastals throughout the room, each with a thick, heavy tome on it. When Rolf touched one of the books, it began to levitate, along with many of the books on shelves around them. Time to roll for initiative!
After a battle that destroyed a third of the library, Vel'Krathis discovered the trick to using the indexes - simply ask for the book you want in the Infernal language. Turns out it's really easy!
Here's the monster. I'll talk a little about how I constructed it afterwards.

Swarm of Books

Large swarm of Tiny constructs, unaligned

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Armor Class 14

Hit Points 67 (9d10+18)

Speed 0 ft., fly 60 ft. (hover)

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Str 14 (+2) Dex 18 (+4) Con 15 (+2) Int 1 (-5) Wis 3 (-4) Cha 1 (-5)

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Damage Resistances bludgeoning, piercing

Damage Vulnerabilities fire

Damage Immunities poison, psychic

Condition Immunities blinded, charmed, deafened, exhaustion, frightened, paralyzed, petrified, poisoned, prone, restrained, stunned

Senses blindsight 60 ft. (blind beyond this radius), passive Perception 6

Languages ----

Challenge 4 (1,100 XP)

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Antimagic Susceptibility. The swarm is incapacitated while in the area of an antimagic field. If targeted by dispel magic, the swarm must succeed on a Constitution saving throw against the caster's spell save DC or fall unconscious for 1 minute.

False Appearance. While the swarm remains motionless, it is indistinguishable from an ordinary library.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny construct. The swarm can't regain hit points or gain temporary hit points.

Actions

Flurry of Pages. +6 to hit, reach 0 ft., one target in the swarm's space. Hit: 22 (4d10) slashing damage, or 11 (2d10) slashing damage if the swarm has half of its hit points or fewer.

Optional Traits

Master Index. The swarm is controlled by a single book at its center. Attack rolls made targeting the master index have disadvantage, but any attack that hits the master index is a critical hit. A DC 15 Wisdom (Perception) check allows a character to notice the existence of a master index.

Though I built this monster before the Dungeon Master's Guide guidelines were out, I followed essentially the same procedure as outlined in that book on pages 274-281. After the guidelines came out, I rebuilt the monster using them, ending up with the stats you see above. Here's the process, step by step.

1. Name. I started with the 4E Book Swarm monster, although all I really kept was the concept. I'm a perfectionist, so I changed it to match the other swarms from the Monster Manual (Swarm of Bats, Swarm of Ravens, etc.).
2. Size. The Swarm of Books probably takes up about as much space as an ogre, so Large it is! Wait, but...
3. Type. Swarms in the Monster Manual are templated "Medium swarm of Tiny beasts," so to match that I had to follow the same formula. Other animated objects are Constructs, and a book is clearly smaller than a Small halfling, so "Large swarm of Tiny constructs" it is!
4. Alignment. Because the books are animated by magic, the Swarm of Books is unaligned, matching other essentially mindless constructs.
5. Ability Scores and Modifiers. Thinking about what I want the swarm to be good at, I assigned stats accordingly. A bunch of magically animated books should be somewhat strong, fast-moving, and resilient (there are a lot of them!). I borrowed the mental stats from the Animated Armor - constructs ought to have roughly the same level of intelligence.
6. Expected Challenge Rating. My party of adventurers was level 5, but I figured they might face multiple swarms at once (two CR 5 creatures is nearly a deadly encounter for 5 level 5 characters). I started with an expected Challenge Rating of 4, giving a proficiency bonus of +2.
7. Armor Class. Looking at the Monster Statistics by Challenge Rating table*, a CR 4 creature has an average AC of 14, which happens to be 10 + the Swarm of Book's dexterity modifier. Perfect!
8. Hit Points. This one was a little tricker. I knew I wanted to give the Swarm some damage resistances, but that would change the number of hit points to apply. So let's move on to step 9...
9. Damage Vulnerabilities, Resistances, and Immunities. Essentially these are assigned when it logically makes sense. Books are clearly vulnerable to fire, but according to the DMG this small weakness doesn't change Challenge Rating.
   Looking at the swarms in the Monster Manual, I noticed that nearly all of them had resistance to the three weapon damage types (bludgeoning, piercing, and slashing). I decided to give the Swarm of Books resistance to bludgeoning and piercing damage, but not slashing. This gives the players a "minigame" - if they figure out the right weapons to use, they can deal more damage. However, since this covers two of the three common weapon damage types, it multiplies HP by 2 (based on our expected challenge rating of 4) - we'll get to this later. I could have made magic weapons able to bypass this resistance, but I decided not to.
   Finally, the Swarm of Books needs some immunities. I again borrowed from the Animated Armor's list of immunities, covering psychic and poison - things an animated object would logically be immune to.
   Phew! Finally, we can calculate hit points. Because we have a resistant monster, we'll want to aim for half the usual number of hit points for a CR 4 monster - that's about 120*, so we'll want roughly 60 HP. Since Large creatures use d10 hit dice (average 5.5), and the Swarm of Books has +2 from its Constitution, we can assign it 8d10+16 hit dice for an average of exactly 60 HP.
   Honestly, though? That seems a little low. We already know the Book Swarm's defensive CR will be right on target, so let's bump it up a notch - we can drop the offensive CR to match our target. After all, many parties will have an easy time burning through that low HP with fire and slashing attacks. If we bump the HD to 9d10+18, we hit an average HP of 67 and an adjusted HP of 134, making this monster a defensive CR 5.*
10. Attack Bonuses. This part is easy. If the Book Swarm attacks with its Dexterity (+4), we add the +2 Proficiency bonus for an attack bonus of +6. This is one over the expected value for CR 4*, but we won't worry about it for now.
11. Damage. Now comes the interesting part. Looking at the swarms in the Monster Manual, we can see that they all have one big attack that deals a lot of damage, which is halved when the swarm goes to half hit points. We'll design the Book Swarm similarly. Since the DMG tells us to consider damage output over the course of three rounds (after which the monster is presumably dead and unable to deal damage), we'll assume that the Book Swarm drops to half HP after the second round for the purposes of calculating damage.
    Since our Defensive CR is 5, we want an offensive CR of 3 to match and average to 4. Because our attack bonus is two higher than expected (bumping the CR up 1), the damage ought to be accordant with that for CR 2 - that is, 15-20.* Let's set the Swarm of Books' output at 4d10, which averages to 22. Remember that this drops to half in the third round, so we calculate an average of (22+22+11)/3 = 18.3, which rounds to 18. Perfect!
12. Save DCs. Since our monster doesn't have any abilities with save DCs, we can skip this step.
13. Special Traits, Actions, and Reactions. On the other hand, a Swarm of Books is quite different from your ordinary monster, which we emphasize with a number of special traits. Looking at the Animated Armor again, we should keep the Antimagic Susceptibility (magically animated objects are vulnerable to dispel magic) and False Appearance traits, neither of which change the challenge rating according to the table on page 280-281. I greatly appreciate the phrasing of False Appearance here, contributed by jadrax on the ENWorld forums. We'll also want to give this creature the Swarm trait, which mostly allows it to move through small spaces.
    For some extra fun, I created an optional trait for this monster. The Master Index trait allows a perceptive (or smart) player to "attack its weak point!" Although it allows for a lot of damage dealt, disadvantage is a harsh enough penalty to make this trait neutral in terms of CR.
14. Speed. In line with the Flying Sword, I gave the book swarm a 0 ft. movement speed on the ground, reasoning that the only way books could move is to fly - I wanted them to be maneuverable, so I gave a large speed. Since the monster doesn't have a ranged attack (and thus must engage in melee to deal damage), flying offers no change to the monster's CR.
15. Saving Throw Bonuses. I elected to skip those, figuring a low-CR monster was unlikely to have any special capability to dodge attacks. Though the low Wisdom might make it vulnerable, the Book Swarm is unlikely to be the target of many attacks.
16. Final Challenge Rating. Now it's time to calculate the Swarm of Books' final CR! Of
    Defensive CR is fairly straightforward. Given extensive resistances at a target CR of 4, we double the HP (based on the p. 277 table), then read down the column and find the monster's HP sitting at CR 5.* Since the Armor Class isn't different by more than a point, it offers no change to the defensive CR.
    For offensive CR, we calculate the monster's three-round damage output (see step 11) at 18, which lands us in CR 2. However, since there's a two-point difference in suggested attack bonus, we bump the offensive CR to 3.*
    Averaging the two values, we've created a CR 4 monster. Congratulations to us!
17. Skill Bonuses. A flying swarm of books isn't likely to be very skillful. We won't worry about this.
18. Condition Immunities. On the other hand, an animated monster is . Referencing the Animated Armor yet again, we see that it is immune to the blinded, charmed, deafened, exhaustion, frightened, paralyzed, petrified, and poisoned conditions. All of these make sense, so we simply copy the list. Looking at swarms, they all seem to be immune to the prone, restrained, and stunned conditions, so let's add those to the list.
19. Senses. Working from the Animated Armor yet again, we can give the Swarm of Books blindsight at a 60 foot radius (it has no eyes, so wouldn't use light to see). Passive Perception is based on Wisdom, so 10 - 4 = 6. 
20. Languages. The Swarm of Books is a nonspeaking monster. And we're done!

*All of these values are derived from the Monster Statistics by Challenge Rating table on page 274 of the DMG.

Next week, I hope to be back with a different monster building excercise. Of the following options, which would be the most helpful?

• A spellcasting monster
• Assessing the CR of a NPC (built like a PC)
• Making simple changes
• Something else!