Echolocation: The swarm can't use its blindsight while Deafened.
Keen Hearing: The swarm has advantage on Wisdom (Perception) checks that rely on hearing.
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 bat. The swarm can't regain hit points or gain Temporary Hit Points.
Magic Resistance: The swarm has advantage on saving throws against spells and other magical effects.
Bites: Melee Weapon Attack: +7 to hit, reach 0ft., twotargets inside the swarm’s space. Hit: 7 (2d6) piercing damage plus 7 (2d6) fire damage, or 3 (1d6) piercing damage and 3 (1d6) fire damage if the swarm has half of its hit points or fewer.
Ok, so I'm not an expert in using swarms, I've used them only once a few years back in 3.5e. I was thinking this as a kind of low-level mini boss, maybe summoned by some kind of cult. What do you think? Is it too weak, too strong? I've bolded in the Bites, as a Large creature, it can have more targets inside its space and thus can make a "double attack". It's mainly a buffed up version of the Swarm of Bats, with the classic traits for Fiends (immunity to fire, poison, poisoned; resistance to cold and magic resistance).
Edit: changed damage from 3d6->2d6, and the halved damage from 2d4->1d6, hp changed from 104->78
With immunity to fire and poison, the swarm has the effective hit points of CR 10 monster. Other than lowering the hit points, I strongly suggest looking at the section in the Dungeon Master's Guide, Creating a Monster. While there are no rules in there for swarms, it still give amazing guidelines for the creation of monsters.
"The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read." - Terry Pratchett
ok, so yeah, I've checked with the Barbed Devil (a fellow devil CR5, which has similar resistances and immunities, and I've figured that you're right... If I want to make it this resilient (the endurance is more or less the same as Barbed Devil, as the latter has a little more AC, a little more hp, but lacks some of the immunities (grappled, paralyzed etc), and is not resistant to magical/silver physical damage (which does rarely make the difference for a CR 5 creature as liv. 2 PGs are not supposed to have magic weapons (consider it's mainly intended for boss combat). The main thing is, the swarm does about double the damage.
so, I think I'll stick to that defense for now, but I'll reduce the damage accordingly. Considering that the swarm does not have available a ranged attack and loses half its damage when under 50% hp, but it has better accuracy than the barbed devil I think that reducing it to 2d6+2d6 should be about right. Consider also, the Barbed devil is intelligent, and has MUCH better saving throws.
"The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read." - Terry Pratchett
I've changed the hp to 78, so now it's defensive CR is 6 (78*2=156, which is CR 6), and the offensive CR is 4 (28avg damage, has higher accuracy but loses half damage at half hit points).
Hellbat Swarm (prof. 3)
Large swarm of tiny Fiends, lawful evil_____________________________________________________
Armor class 14
Hit points 78 (12d8+24)
Speed 30 ft., fly 60 ft. ____________________________________________________________________________
STR DEX CON INT WIS CHA
7(-2) 18(+4) 14(+2) 3(-4) 12(+1) 4(-3) ___________________________________________________
Skills Perception +4, Stealth +8
Damage Resistances Cold, Bludgeoning, Piercing, Slashing
Damage Immunities Fire, Poison
Condition immunities Charmed, Frightened, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained, Stunned
Senses Blindsight 90ft., passive Perception 14
Challenge 5 (1800 XP)_________________________________________________________________________________________
Echolocation: The swarm can't use its blindsight while Deafened.
Keen Hearing: The swarm has advantage on Wisdom (Perception) checks that rely on hearing.
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 bat. The swarm can't regain hit points or gain Temporary Hit Points.
Magic Resistance: The swarm has advantage on saving throws against spells and other magical effects.
Actions_____________________________________________________________________________________________
Bites: Melee Weapon Attack: +7 to hit, reach 0ft., two targets inside the swarm’s space. Hit: 7 (2d6) piercing damage plus 7 (2d6) fire damage, or 3 (1d6) piercing damage and 3 (1d6) fire damage if the swarm has half of its hit points or fewer.
Ok, so I'm not an expert in using swarms, I've used them only once a few years back in 3.5e. I was thinking this as a kind of low-level mini boss, maybe summoned by some kind of cult. What do you think? Is it too weak, too strong? I've bolded in the Bites, as a Large creature, it can have more targets inside its space and thus can make a "double attack". It's mainly a buffed up version of the Swarm of Bats, with the classic traits for Fiends (immunity to fire, poison, poisoned; resistance to cold and magic resistance).
Edit: changed damage from 3d6->2d6, and the halved damage from 2d4->1d6, hp changed from 104->78
With immunity to fire and poison, the swarm has the effective hit points of CR 10 monster. Other than lowering the hit points, I strongly suggest looking at the section in the Dungeon Master's Guide, Creating a Monster. While there are no rules in there for swarms, it still give amazing guidelines for the creation of monsters.
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Astromancer's Homebrew Assembly
"The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read." - Terry Pratchett
ok, so yeah, I've checked with the Barbed Devil (a fellow devil CR5, which has similar resistances and immunities, and I've figured that you're right... If I want to make it this resilient (the endurance is more or less the same as Barbed Devil, as the latter has a little more AC, a little more hp, but lacks some of the immunities (grappled, paralyzed etc), and is not resistant to magical/silver physical damage (which does rarely make the difference for a CR 5 creature as liv. 2 PGs are not supposed to have magic weapons (consider it's mainly intended for boss combat). The main thing is, the swarm does about double the damage.
so, I think I'll stick to that defense for now, but I'll reduce the damage accordingly. Considering that the swarm does not have available a ranged attack and loses half its damage when under 50% hp, but it has better accuracy than the barbed devil I think that reducing it to 2d6+2d6 should be about right. Consider also, the Barbed devil is intelligent, and has MUCH better saving throws.
The swarm still has a CR of 7 at full health. I strongly encourage you to half the hit points.
Tooltips | Snippet Code | How to Homebrew on D&D Beyond | Subclass Guide | Feature Roadmap
Astromancer's Homebrew Assembly
"The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read." - Terry Pratchett
I've changed the hp to 78, so now it's defensive CR is 6 (78*2=156, which is CR 6), and the offensive CR is 4 (28avg damage, has higher accuracy but loses half damage at half hit points).