Ochre Jelly

An ochre jelly on the party's side requires a keeper, almost always a wizard, alchemist, or grim caretaker who has fed and contained one for years and uses it for a specific industrial purpose: cleaning carcasses for an apothecary, dissolving organic locks on a sealed crypt, processing the rendered fat that some ritual requires. The party either inherits the keeper's notes or hires the keeper as part of a downtime arrangement. The jelly is never the ally; the keeper is.

Use the jelly to clear organic obstacles in a controlled way. Pour it into the gut of a slain otyugh to render the venom sacs safe. Drop it onto a patch of strangling vines to free a trapped wagon. The pseudopod's 12 average acid plus Blindsight makes this a dependable tool against soft, non-metallic, non-living-creature problems, and the keeper's bond keeps it from drifting toward the party's own boots.

Do not bring it to a fight. The jelly cannot tell allies from enemies, and any attempt to weaponize it during initiative is one Pseudopod swing at the closest warm body, friend or foe. Worse, every time an ally swings a sword at an enemy beside the jelly and misses, you are one bad roll away from a Split reaction triggered on a stray strike, and now the party has two jellies in the room they did not plan for.

The keeper should be visibly nervous around their own creature. Players will treat any ooze with the respect it deserves once they see the person who owns it flinch.

An ochre jelly is a math problem the party will keep solving until they understand it. The Pseudopod hits at +4 for 12 average acid, AC 8 means almost everything connects, and 52 HP looks short for CR 2. The trick is Split: any time the jelly takes lightning or slashing damage while at 10+ HP and Large or Medium, or any time it becomes Bloodied, it cleaves into two new ochre jellies, one size smaller, sharing the parent's remaining HP rounded down. The fighter who opens with a 14-damage greatsword swing has just turned one jelly into two, and the wizard who follows up with Lightning Bolt has turned two into four.

Open in a tight corridor or a vertical shaft. Spider Climb plus a 20 ft. climb speed plus Amorphous (the jelly squeezes through 1-inch gaps) means the ochre jelly fights from the ceiling and the walls and the floor at once, and a party that scatters into a side passage finds the jelly oozing under the door behind them. Blindsight 60 ft. and Pass Perception 8 means the jelly knows where everyone is in the dark, so Darkness and similar tricks do nothing.

Damage typing is the whole encounter. Slashing damage and Lightning damage are immune (Slashing) or trigger Split (Lightning), so a paladin with a longsword and a sorcerer with Witch Bolt are both actively making the encounter worse. Bludgeoning, piercing, fire, cold, force, radiant, necrotic, psychic, thunder all work normally. A clever party figures this out by round two; a stubborn party that keeps hacking with axes loses a healer to acid by round four. Don't tell the players. Let the table-talk happen.

The jelly does not retreat. Intelligence 2 and no instinct for survival mean it eats until every blob is dead. Once a jelly is reduced to 9 HP it can no longer Split, so the cleanup phase is the players catching the small ones one at a time and stomping them out.

Place the encounter under a previous fight's leftover bones. The jelly is what is still there, slowly, after the dragon left.