Birthday Cake Sludge

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

Corrosive Form. A creature that touches the pudding or hits it with a melee attack while within 5 feet of it takes 5 (1d10) acid damage. Any nonmagical weapon made of metal or wood that hits the pudding 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 pudding is destroyed after dealing damage.

The pudding can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

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

Multiattack. The sludge makes two Slam attacks

Slam. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 13 (3d8) 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.

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 7 (1d10 + 1) acid damage, and the target is grappled (escape DC 11). Until this grapple ends, the target is restrained, and the sludge can't bite another target.

Swallow. The sludge makes one bite attack against a Small or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the sludge, and it takes 12 (5d4) acid damage at the start of each of the sludge's turns. The sludge can have only one target swallowed at a time. If the sludge dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

The Birthday Cake Sludge can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary Actions are regained at the start of each turn.

Slam. The sludge makes a slam attack.

Congest (Costs 2 Actions). The sludge deals 20 (8d4) acid damage to any creature it has swallowed.

Infernal Candles (Costs 3 Actions). The sludge flares its candles. Each creature of the sludge's choice that is within 40 ft. of the sludge and aware of it must succeed on a DC 14 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the sludge's Infernal Candles for the next 24 hours.