Anvil Rats

Mob Fighting. Functions as one creature representing many individuals. When reduced to half hit points or fewer, they lose their Brutal Swarm trait.

Brutal Swarm. On a successful melee hit, the Anvil Rats deal an extra 3 (1d6) bludgeoning damage from sheer numbers and force.

Ash Hardened. They have resistance to fire damage (from living near forges and furnace vents).

Multiattack. The Anvil Rats make two Scrap Cleaver attacks.

Scrap Cleaver. Melee Weapon Attack: +4 to hit, reach 5 ft., one target.
Hit: 9 (2d6+2) slashing damage.

Coal Dust Toss (Recharge 5–6). The Anvil Rats scatter hot ash or grit in the air. Each hostile creature within 10 feet must succeed on a DC 12 Constitution saving throw or be blinded until the start of its next turn.