Stone Golem

An allied stone golem is a bound guardian, almost always inherited rather than friended. The party found the command word in a wizard's notes, broke a sigil on the floor, or completed the ritual the dead artificer never finished. The golem now follows simple instructions to the letter. It does not improvise, and it interprets orders without nuance. "Guard the door" means the door, not the room behind it.

In combat on the party's side, point and let it work. Two attacks per round at +10 to hit (Slam, 5 ft. reach, 24 average damage with the bludgeoning and force together) or +9 to hit at range (Force Bolt, 120 ft., 22 force) makes it a reliable damage anchor that the party plays around. Slow as a bonus action on a 5-6 recharge is a major control tool the party probably can't replicate. Drop it on the enemy back rank and watch the encounter shape change. AC 18 and 220 HP means the golem soaks the alpha strike that would have killed the wizard.

The price is the limits. The golem cannot climb a rope or swim (it weighs about 1,000 pounds), it does not fit through narrow doorways, and it will attack a friend the second somebody slips up and orders it to. The party who gives a careless command in earshot of the golem learns this fast. Carry the command word on a card. Lose it and the golem stops where it is.

Have the golem refuse to leave the threshold of its original creator's tower. The party can borrow it for a day, but it walks itself home at sunset.

A stone golem is a slow attrition fight with a big surprise. The party will look at AC 18, 220 HP, and 30 ft. of speed and assume they can kite it forever. They cannot. Force Bolt is a +9 ranged attack out to 120 ft. for 22 force damage on a hit, which means the party's archer is in range every round and the wizard cannot hide behind a low wall. The golem doesn't have to chase. It can stand in a doorway and pillar the back rank across the room.

Open with a Slow cast if it's available. Bonus action, DC 17 Constitution save (Recharge 5-6), and on a fail the targets in a 40-foot cube have their speed halved, can take only one action or bonus action per turn, lose reactions, and take a -2 to AC and Dex saves. That single bonus action ruins a martial-heavy party for several rounds. Follow with Multiattack: two attacks chosen from Slam (+10 to hit, 5 ft. reach, 15 bludgeoning plus 9 force) or Force Bolt (+9 to hit, 120 ft., 22 force). In melee with a fighter, two Slams is 48 average. With the wizard at 80 ft. and the cleric at 30 ft., one Force Bolt at each is 44 force damage and bypasses every common resistance.

Magic Resistance plus Damage Immunity to Poison and Psychic plus Condition Immunity to Charmed, Exhaustion, Frightened, Paralyzed, Petrified, and Poisoned shut down most caster control. Hold Monster, Banishment, and any poison spell are wasted. Dispel Magic does nothing. The party has to grind 220 HP down with damage. The golem does not retreat. It was given an order, it follows that order, and it stops only when the order is satisfied or when it is broken. Run it as a problem the party has to solve, not an enemy that hates them.

Have the golem speak only once in the encounter. A single Common phrase, the original command its creator gave it, repeated in a flat voice as it advances. The party will remember the line.