Black Icer

Amorphous. The black icer can move through a space a narrow as 1 inch wide without squeezing.

Slick Surface. Black icers have extremely slick surfaces when hardened. Any creature stepping on a black icer must make a DC 15 Dexterity saving throw or slip and fall on top of the black icer. The black icer may engulf the creature as a reaction.

Transparent. Black icers are difficult to see when on a smooth surface and even when they settle into a patch of “ice” on natural ground they look no different than regular ice. Even when in plain sight, it takes a successful DC 17 Wisdom (Perception) check to spot a black icer that has neither moved nor attacked. A creature that tries to enter the black icer’s space while unaware of the black icer is surprised by the black icer.

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage. 

Engulf. The black icer moves up to its speed. While doing so, it can enter Large or smaller creatures’ spaces. Whenever the black icer enters a creature’s space, the creature must make a DC 15 Dexterity saving throw. On a successful save, the creature does not slip and may enter the nearest 5-foot space outside of the black icer’s spaces. On a failed save, the creature takes 10 (3d6) cold damage and is engulfed.
The engulfed creature can’t breathe, is restrained, and takes 21 (6d6) cold damage at the start of each of the black icer’s turns. The black icer cannot move while it has engulfed a creature (although it may attack others with its pseudopods) and it can only engulf one creature at a time.
An engulfed creature can try to escape by taking an action to make a DC 16 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the black icer.