in “Dawn of the Overmind“, below the Undercity of Penumbra, deeper yet, there are the Tunnelscapes dominated by warring grimlock tribes each sure they are in the right about the interpretation of a shared religious article – the Gyor (“grey“) and the Hiao (“green“).
One of the tribes has learned to “tame” velvet ants – a type of ant with a velvet-like growth on their shell. The ants serve as a food source for the grimlocks when prepared correctly. The adventure has a statblock for them but it’s doubtful the PCs will come into conflict with the swarm. Here is the 5e version.
SWARM OF VELVET ANTS
Medium swarm of Tiny beasts, unaligned
Armor Class 12 (natural armor)
Hit Points 22 (5d8)
Speed 20 ft., climb 20 ft.
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained, Stunned
Senses blindsight10 ft., passive Perception 8
Challenge 1/2 (100 XP)
Swarm 2.0. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny beast. The swarm can’t regain hit points or gain temporary hit points. A swarm takes half damage from attacks that use an attack roll and double damage from effects that require it to make a saving roll. Creatures that are not swarms are impaired while they remain in the swarm’s space or within 5 feet of it and have disadvantage on attack rolls and Constitution saving throws to maintain concentration. The area of the swarm is considered difficult terrain.
Poison Bites. All targets in the swarm’s space take 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half of its hit points or fewer and 11 (2d10) poison damage. The creature must make a DC 10 Constitution saving throw or become poisoned for 1 minute. While poisoned the target is wracked with pain and has disadvantage on attack rolls and Constitution saving throws to maintain concentration spells.