Gray Ooze

A gray ooze on the party's side is genuinely odd, but the wizard-with-a-laboratory framing carries it. A spellcaster has bound the ooze in a sealed flask, feeds it scraps of nonmagical metal, and uses it to dissolve specific obstacles. The party either inherits the flask from a dead patron or buys it off a retiring alchemist who throws in the warning that the flask has held for forty years and might not hold for forty-one.

Treat the ooze as one-shot consumable terrain. Pour it on a portcullis chain, a strongbox lock, an iron grate, and walk away. Corrosive Form lets it eat through 2 inches of nonmagical metal or wood in a round, which means most dungeon doors fall in two to three rounds of patient ooze. The party trades a flask charge for a problem solved without explosives or noise. Three or four uses out of the flask, then the ooze either dies, escapes, or grows large enough that the binding fails and the campaign has a new problem.

Do not let the ooze fight on the party's side in combat. It cannot tell friend from foe, has Intelligence 1, and Pseudopod is an indiscriminate splash of acid that will hit whoever is closest. If the players try to weaponize it during initiative, narrate the ooze surging toward the nearest warm body, which is usually the rogue who threw the flask. The ooze is a tool, not a teammate, and the moment the party forgets that is the moment the ooze becomes an adversary again.

The flask should always feel one bad jostle from breaking. Roll a die in front of the players when they take a hit while carrying it, even if nothing comes of the roll.

A gray ooze is a tax on the party's gear, dressed up as a low-CR encounter. The damage line looks modest (Pseudopod at +3 for 10 average acid), and 22 HP at AC 9 means the fighter eventually wins. The actual cost of the fight is Corrosive Form: every nonmagical weapon that hits the ooze takes a cumulative -1 attack penalty per strike and is destroyed at -5, every nonmagical arrow is consumed on impact, and every Pseudopod hit shaves a -1 off nonmagical armor that destroys it at AC 10. Bring a gray ooze to the table and the party leaves poorer.

Disguise it. Stealth +5 against a passive Perception of 8 means the ooze is bad at hiding from anyone looking for it, but it does not need to hide so much as look like the floor. Run it as a wet patch on stone, a sheen down a corridor wall, a puddle near a corpse the party is searching. Blindsight 60 ft. means the ooze knows exactly where the party is even in pitch dark, so let it strike the moment somebody steps adjacent. The first attack should land before the player even rolls initiative, because they did not see a creature, they saw a stain.

Once combat starts, the ooze does the only thing it can do: pseudopod, pseudopod, pseudopod. Amorphous lets it slide through a 1-inch gap, so a party that retreats up a flight of stairs and slams a door is not safe. It oozes under. Target the heavily armored PC on every turn so the armor penalty stacks fast. Resistance to Acid, Cold, and Fire makes elemental cantrips half-useful at best, and the immune-conditions list (Charmed, Frightened, Grappled, Prone, Restrained, more) shuts down most of the party's non-damage answers. Force damage, radiant, lightning, slashing all work normally; let the players figure that out.

The ooze does not retreat. It has Intelligence 1 and no instinct for self-preservation. It eats until it is dead. End the encounter when it pops, not before.

Have the party hear the squelch from the next room before they enter. Players who rush in past a wet sound deserve what they get.