Fire Form

The creature can move through a space as narrow as 1 inch wide without squeezing. Targets that touch the creature or hits it with a melee attack while within 5 feet off it takes X fire damage. In addition, the creature can enter a hostile target's space and stop there. The first time it enters a target’s space on a turn, that target takes Y fire damage and catches fire; until someone takes an action to douse the fire, the target takes Z fire damage at the start of each of its turns. (Fire Elemental, pp. 125, MM)