Troll

The Troll never dies until someone brings fire

How to run it. 94 Hit Points, AC 15, three Rends a turn at +7 for 2d6 + 4 each, and reach 10 feet. Against AC 16 each Rend hits 60% of the time, and at least one lands 94% of the time. It also regains 15 Hit Points at the start of every turn.

The rule that matters. Knocking a troll to 0 does not kill it. It dies only if it starts its turn at 0 and fails to regenerate, and the only thing that stops regeneration is Acid or Fire damage taken since its last turn. A party with no fire or acid can drop it every round and it stands up every round. A torch counts: an attack with a lit Torch deals 1 Fire damage, which is all it takes.

Losing limbs. If the troll ends a turn Bloodied (47 or fewer) and took 15 or more Slashing damage that turn, a limb falls off and fights on its own. A fighter landing two longsword hits (1d8 + 3 each) makes 15 about 56% of the time. The limbs grow back the next time it heals, so severing without fire only adds enemies.

The common mistake is hiding the weakness or giving it away. Let the players see it: a wound closing, a burnt scar that stays. Then let them work it out.

For players. Save the Fire Bolt for the round it hits 0. Fire earlier still helps, because a turn without 15 free Hit Points is a turn you gained.

The human angle. Karl Duncker's 1945 problem-solving studies showed people struggle to use a familiar object in an unfamiliar way; he called it functional fixedness. A troll is that test at the table. The answer sits in someone's pack as a light source, and the moment a player sees the torch as a weapon is the best moment of the fight.

Source: Duncker, K. (1945). On problem-solving. Psychological Monographs, 58(5), i–113. doi:10.1037/h0093599