Storm Colossus (5e Creature)
From D&D Wiki
Gargantuan elemental, chaotic neutral
Air Form. The colossus can enter a hostile creatures space and stop there. For each of the colossus's turns spent in a creature's space the creature must make a DC 17 Strength saving throw or be knocked prone. It can move through a space as narrow as 1 inch wide without squeezing.
Action Name. Melee Weapon Attack: +X to hit, reach 5 ft., one target. Hit: X (1dX + X) damtype damage.
The can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The regains spent legendary actions at the start of its turn.