Blood Elemental

Invisible in Blood. The blood elemental is invisible while fully immersed in blood.

Water Form. The blood elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 foot wide without squeezing.

Freeze. If the blood elemental takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.

Multiattack. The elemental makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (1d6 +4) piercing damage. 

Blood Jet (Recharge 4-6). A jet of blood forming a line 50 feet long and 5 feet wide blasts out from the elemental in a direction it chooses. Each creature in the line must make a DC 15 Dexterity saving throw. A creature takes 17 (3d8 + 4) bludgeoning damage on a failed save and is pushed to the end of the line. There is a 25 percent chance that any creature that fails its save takes an additional 7 (1d6 + 4) piercing damage and is pinned to the first object it hits. The creature will be under the effects of the restrained condition. A creature that succeeds on the save takes half as much damage and is not pushed.