Puddle of Boiling Tar

Undead Technicality. Any effect that would affect undead creatures also affects the tar.

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

Boiling Body. The tar ignites any flammable objects it comes into contact with that aren't being worn or carried. 

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

False Appearance. While the tar remains motionless, it is indistinguishable from an inanimate puddle of boiling tar.

Adhesive. The tar adheres to anything that touches it. While a creature is adhered to the tar, it takes 9 (2d8) fire damage at the end of each of the tar's turns. A creature adhered to the tar is also grappled by it (escape DC 15). The tar can grapple any number of creatures in this way. Ability checks made to escape this grapple have disadvantage. 

Pseudopod. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage plus 18 (4d8) fire damage. The target is subjected to the tar's Adhesive trait.

Tarball. Ranged Weapon Attack: +1 to hit, range 20/100 feet, one target. The tar loses 4 (1d8) hitpoints. This attack fails and the tar does nothing with its action if this would cause it to die. Hit: 13 (3d8) fire damage at the start of each of the target's turns. A creature can end this damage by using its action to make a DC 15 Strength check remove the tar from itself. Miss: A patch of hot tar covers a randomly determined 5-foot-square section of wall or floor within 5 feet of the target. This tar is difficult terrain. A creature that comes into contact with the tar or ends its turn in the same space as the tar takes 5 (1d10) fire damage.

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