Flesh Golem

A flesh golem on the party's side is the wizard's bodyguard, the necromancer NPC's gift, or a curiosity recovered from a collapsed laboratory and nominally bound to the party's cleric. The golem understands Common plus one other language but cannot speak. Treat it as a mute servant with a two-word vocabulary of obedience.

In combat it is a tank that the party cannot heal conventionally. 127 HP, immunity to Poison, Magic Resistance, and a respectable Slam means it can stand in a doorway for several rounds while the party works behind it. The lightning rider on Slam means an enemy with lightning resistance shrugs off the secondary damage, but most things take the full 17. Once the golem drops below 63 HP, the Berserk roll starts every turn, and the party needs to be ready to either drop it or distance themselves. The golem will turn on the cleric who has been healing it the entire fight.

Lightning Absorption is the trick. A friendly storm sorcerer can keep the golem topped up by burning lightning spell slots into it. This is one of the few healing methods in the game that scales with caster level, and it's worth telegraphing if anyone in the party can cast lightning damage. A shocking grasp from a familiar every round is a noticeable trickle of healing.

Aversion to Fire is the constant logistical problem. Fire-using enemies, torches in close quarters, a friendly fire-domain cleric, all of these debuff the golem. Position carefully. Once the work is done, the golem stops, faces the wall, and waits for the next order. It does not converse. It does not ask. The party should feel uneasy about how easy it is to live with.

A flesh golem is a slow tragedy with 127 HP. AC 9 means the party hits it almost every swing, but Magic Resistance plus immunity to Lightning and Poison plus immunity to half the disabling conditions means most of the party's actual answers don't work. The fight is grinding, sad, and physical. Sell the tragedy. The golem was someone, or several someones, before it was a golem.

Multiattack is two Slams at +7 for 17 average each (13 bludgeoning plus 4 lightning). That's 34 damage on full hits, enough to drop a level 5 squishie in one round. The lightning rider is the trap: if a party member is wearing a ring of lightning resistance or has cast protection from energy, they shrug. If they tried to hit the golem with a lightning spell, they just healed it. Lightning Absorption converts every point of lightning damage straight into HP, so the warlock who opens with eldritch blast and the cleric with a chain lightning scroll have actively made the fight harder.

Berserk is the lever. Once the golem is bloodied (63 HP), every turn starts with a 1-in-6 roll. On a 6 the golem attacks the nearest creature, friend or foe, until destroyed or healed above bloodied. If the golem has a creator or handler in the scene, that NPC can spend an action on a DC 15 Charisma (Persuasion) check to calm it for one turn. Use this. A creator desperately whispering to their berserk creation while the party closes in is more interesting than a slugfest.

Aversion to Fire is the party's other answer. Any Fire damage gives the golem Disadvantage on attacks and ability checks until end of next turn. A single firebolt every round from a cantrip caster effectively halves the golem's damage output. Reward the player who figures this out.

The golem doesn't retreat. It cannot. It either finishes the order it was given or it falls, and in the silence after the fight the party should hear something that sounds almost like relief.