Cobbleswarm

False Appearance. While the swarm remains motionless, it is indistinguishable from normal stones.

Shift and Tumble. As a bonus action, the swarm can push a prone creature whose space it occupies 5 feet.

Shifting Floor. Whenever the swarm moves into a creature's space or starts its turn in another creature's space, that other creature must make a successful DC 13 Dexterity saving throw or fall prone. A prone creature must make a successful DC 13 Dexterity (Acrobatics) check to stand up in a space occupied by the swarm.

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 stone. The swarm can't regain hit points or gain temporary hit points.

Stings. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half its hit points or fewer.