Another conversion for my own reference. Lifted from “Through Ultan’s Door” and dropped in as a random encounter for Maze of the Blue Medusa. Abbreviated 5e conversion below.
Before the creature strikes it gives off an aura of dream visions. Use any surreal art as an inspiration of what the PCs see. Each must make a DC 12 Charisma saving throw or be surprised when the oneiric pudding attacks.
Oneiric Pudding
Amorphous. The pudding can move through a space as narrow as 1 inch wide without squeezing.
Dream Form. A creature that touches the pudding or hits it with a melee attack while within 5 feet of it takes 4 (1d8) psychic damage. Any weapon made of metal or wood that hits the pudding seems to corrode – becoming rubbery, or changing form into a clock or some other out of place object. After dealing damage, the weapon takes a temporary and cumulative −1d2 penalty to damage rolls. If its penalty drops to less than its maximum damage number, the weapon is destroyed. For example, a weapon that does 1d6 damage is destroyed after it has accumulated 3d2 penalty. If the weapon survives the battle it returns to normal after the encounter. 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.
Actions
Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 18 (4d8) psychic damage. In addition, armor worn by the target is partly, seemingly, dissolved and takes a temporary and cumulative −1 penalty to the AC it offers. The armor is actually destroyed if the penalty reduces its AC to 10, otherwise, it reverts to normal after the pudding is killed. If the pudding scores a critical hit the ooze flows into the target creatures orifice and it begins to drown. This process normally takes 3 rounds.