Troll

A troll fighting on the party's side is hungry, leashed, and dangerous to be near. Maybe a hag patron lent them one for an evening. Maybe a giant-friendly druid called in a favor. Maybe the party freed a troll from a hunter's pit and now it follows them, mostly. The framing has to make peace with two things: the troll has Int 7 and Cha 7, and the troll is Chaotic Evil. It is not a friend. It is a wolf you have meat for.

In combat, point it at the biggest enemy and stand well back. Charge plus three Rend attacks at +7 dishes 33 damage on a full Multiattack, and Regeneration means it stays in the fight long after a normal CR 5 would be down. Make sure no party member is wielding fire or acid in melee, because the troll will not distinguish friendly burns from enemy ones, and a single careless fireball shuts off its own healing for the next round. Tell the wizard to angle their cones away.

The troll asks for nothing complicated. It wants to eat what dies. It wants to keep what it grabs. If the party tries to take a prisoner the troll has already bitten, there's a problem. Once the fight is over, the troll wanders off to find more food and may not come when called next time. Treat each appearance as a one-shot favor, not a recurring companion.

If the players want to keep it, make them feed it before every encounter. Whose rations, and how much, is a roleplay scene worth running.

A troll is a CR 5 puzzle wearing 94 HP and three claws. The puzzle is Regeneration: 15 HP back at the start of every turn, and the only way to switch it off for a round is fire or acid damage. Players who don't carry torches or alchemist's fire learn the rule the hard way, which is the entire reason this monster exists in the bestiary.

Open with Charge. The Bonus Action gives the troll half its Speed straight at an enemy, and Multiattack puts three Rend at +7 (11 average each, 33 on a full hit) into reach 10 ft. Pick the squishiest target in line of sight and run at them. Trolls are Int 7 and lean on instinct, so they go for whoever's in robes or whoever just hit them hardest. Don't bother with flanking or held actions. The troll closes, swings, and keeps swinging until something burns it.

Loathsome Limbs is your turn-three swing trait. While Bloodied, a turn where the troll takes 15+ slashing severs a limb that drops into its space and acts immediately after the troll's turn as a separate Troll Limb. The troll picks up an Exhaustion level for each missing limb, but the player who brought the slashing build now has another body on the field to deal with. Slashing-heavy parties get punished for their build choice; bludgeoning parties skip the trait entirely. Telegraph this once at the table so the wizard with fire bolts knows why the fighter is suddenly fighting a forearm.

Trolls don't retreat unless something is keeping them on fire. They eat the dead either way. When the players finally drop one, describe the wounds smoking shut for a full round before they go still. The first time a player thinks they killed a troll and watched it stand back up is a moment they tell the rest of the table about for years.