Demon Ichor

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

A creature that touches the ichor or hits it with a melee attack while within 5 feet of it takes 4 (1d8) Acid damage. Any nonmagical weapon made of metal or wood that hits the ichor 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 or wood that hits the ichor is destroyed after dealing damage. The ichor can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

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

Melee Weapon Attack +5 to hit, reach 5 ft., one target. 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.

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