Multiattack

The bone colossus makes two attacks.

Thunderous Slam (Colossus Form Only)

Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 29 (4d10 + 7) bludgeoning damage plus 10 (3d6) thunder damage, and the target must succeed on a DC 18 Strength saving throw or be knocked prone.

Razor Teeth (Swarm Form Only)

Melee Weapon Attack: +12 to hit, reach 0 ft., one target in the swarm's space. Hit: 21 (6d6) piercing damage, or 10 (3d6) piercing damage if the swarm has half its hp or fewer.