Blood Boil

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

Amphibious. The boil can breath air & water.

Corrode Metal. Any nonmagical weapon made of metal that hits the boil corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the boil is destroyed after dealing damage. The boil can eat through 2-inch-thick, nonmagical metal in 1 round.

False Appearance. While the boil remains motionless, it is indistinguishable from an oily pool or wet rock.

Ooze Nature. The boil doesn't require sleep.

Spider Climb. The boil 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: 4 (1d8 + 3) bludgeoning damage plus 10 (3d6) necrotic damage, and the boil regains hit points equal to the necrotic damage dealt. If the target is wearing nonmagical metal armor, its armor is partly corroded 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.