Brain Sludge

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

Spider Climb. The Sludge can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 18 (4d8) acid damage. In addition, nonmagical armor worn by the target is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

Take Over. Once per turn, if the target is within 25 feet, the Brain Sludge can attempt to take over the target's mind. The target must make a Wisdom saving throw or else the Brain Sludge climes into the target, kills their body, devours their brain, occupies the space where the brain used to be, and controls the target's body. This effect is permanent, until the Brain Sludge comes out of the body. Wall Run. If it is standing on rock, it can go into the walls, floor, or ceiling of the cave, and on its next turn, it will emerge within 30 feet of the spot it slid into.

Split. When a Sludge that is Medium or larger is subjected to lightning or slashing damage, it splits into two new Sludges if it has at least 10 hit points. Each new pudding has hit points equal to half the original pudding's, rounded down. New Sludges are one size smaller than the original Sludge.