Hydra

A hydra ally is a stretch and you should name the stretch. The framings that work: a druid's shapechange or summoned ally during a high-tier ritual, a monstrosity that the party fed and quietly tamed during downtime in a swamp campaign, or a guardian beast bound to a temple where the party has taken sanctuary. The hydra is Int 2 and has no listed languages, so this is not a thinking partner. It is a weapon the party has learned to point.

The job in combat is area denial. Park the hydra in a chokepoint (a doorway, a bridge, a 15-foot circle of mud) and let the five Bite attacks per turn plus Reactive Heads turn that space into a place enemies do not walk through. Anyone who tries to flank the party has to deal with up to four opportunity attacks while crossing the threat zone. The hydra cannot be commanded mid-round and will bite whoever is closest, so position it where the closest valid target is the enemy.

The serious risk is the party's own fire. If the cleric drops a Spirit Guardians or the wizard fireballs the chokepoint, the hydra benefits (no head regrowth becomes irrelevant since the hydra is on the party's side and not losing heads to the players). If the players are hitting the hydra by accident with their own AOE, however, the head loss math runs in reverse and the hydra now has more heads, more Reactions, and more damage output. This is funny once and a problem twice. Have the hydra leave when the immediate threat is gone. It will return to whatever pool, ruin, or temple grove it came from, and the party should never assume it follows them home.

Give the hydra a single sound the players know means it is nearby. A wet exhale, a smell of rotten reeds, a particular ripple on the water. When that signal returns three sessions later, the table will remember.

A hydra is the encounter that teaches your party the difference between damage and damage type. Five heads, five Bite attacks per Multiattack at +8 for 10 piercing each, 184 HP, AC 15, walk 40 ft., swim 40 ft. The Multiple Heads trait is the whole fight: every 25 damage in a single turn kills one head, and at the end of the hydra's turn each dead head grows back as two unless the hydra has taken Fire damage since its last turn. The hydra also regains 20 HP when it grows new heads. Reactive Heads gives one extra Opportunity Attack reaction per head beyond the first.

Open in water if you can. Swim 40 ft. plus Hold Breath for an hour means the hydra ambushes from a swamp pool or river by surfacing in the middle of the marching order, and the heads can attack from 10 ft. of reach in every direction. The first round, take five Bite attacks and spread them across the party so every PC has to think about engagement. Reactive Heads makes movement around the hydra expensive: anyone breaking off without Disengage triggers up to four opportunity attacks. The party that doesn't bring fire learns this the slow way.

The interesting decision space is the head economy. If a player hits one head for 25 in a turn, that head dies and the hydra grows two back at end of turn unless someone has dealt fire damage since the hydra's last turn. So the gameplay loop is: do 25 to a head, then have someone (anyone) put 1 fire damage on the hydra before its next turn ends, or the head count goes up. Run this honestly. Let the players discover it. The first time the cleric realizes they need to torch the stump, the encounter becomes a puzzle.

The hydra is immune to Blinded, Charmed, Deafened, Frightened, Stunned, and Unconscious. No save-or-suck. It does not surrender, does not flee, and it has Int 2, so it does not change tactics. It bites until it dies. End the encounter when the last head goes, and let the party stand in the bloody water for a beat.