Shadow Form. The elemental can move through a space as narrow as 1 inch wide without squeezing. When the elemental moves through another creature's space, the target must make a DC 15 Constitution saving throw. On a failiure, the target takes 13 (2d8+4) 2d8+4 necrotic damage, or half of that on a success.
Light Susceptibility.If the elemental moves into bright light it takes 10 (2d6+3) radiant damage, and if it moves into dim light it takes half that. It takes the damage if the light is moved so it hits it as well.
Spreading Darkness.If the elemental kills a creature, it rises as a shadow elemental with half of the hit points of the original. 6d10+2 / 3d10+1 / 1d10 / 1d4 / 1d2