Black Pudding

The monster can move through a space as narrow as 1 inch without expending extra movement to do so.

A creature that hits the monster with a melee attack roll takes damage. Nonmagical ammunition is destroyed immediately after hitting the monster and dealing any damage. Any nonmagical weapon takes a cumulative −1 penalty to attack rolls immediately after dealing damage to the monster and coming into contact with it. The weapon is destroyed if the penalty reaches −5. The penalty can be removed by casting the Mending spell on the weapon.
In 1 minute, the monster can eat through 2 feet of nonmagical wood or metal.

The monster can climb difficult surfaces, including along ceilings, without needing to make an ability check.

. . Nonmagical armor worn by the target takes a -1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10. The penalty can be removed by casting the Mending spell on the armor.

Trigger: While the monster Response: The monster splits into two new monster. Each new monster is one size smaller than the original monster and acts on its Initiative. The original monster's Hit Points are divided evenly between the new monsters (round down).