Iron Golem

An iron golem on the party's side is a vault gift, a contract with a dwarven forge-cult, or a relic the party seized from a dead wizard whose command-words they translated. It is unaligned, mindless, and bound by the original command structure the party has to learn or replace. Treat it as the most dangerous static defense in the campaign, not a companion who walks beside the party on the road.

The single best use is fortress defense. The iron golem stands in one chokepoint and breaks anything that comes through it. AC 20, 252 HP, immunity to Fire, Poison, and Psychic, plus advantage on saves against spells via Magic Resistance. Any siege the party doesn't want to be present for, leave the golem at the door. With a 60 ft. Bladed Arm reach (10 ft. plus the 30 ft. walk pattern) and Multiattack giving two of either Bladed Arm or Fiery Bolt at +12/+10, it murders most things that approach. Save Poison Breath (Recharge 6) for the moment a wave of low-HP attackers clusters in the 60 ft. cone, since 55 average Poison damage on a failed DC 18 Con save will end most of them in one breath.

The catch is Fire Absorption. If the party brings the iron golem to a fight where they intended to use Fireball, Wall of Fire, or any fire damage at all, the golem heals from it. Plan accordingly, and tell the party caster before initiative which damage type they should not be reaching for.

The iron golem is the worst diplomatic ally in the bestiary. It understands Common but cannot speak. Any conversation about why it just gored a guard is one-way, and the party will not enjoy the cleanup if they brought the golem into a city. Keep it in the dungeon where it belongs.

An iron golem is a CR 16 wall that walks. The encounter is not about damage spikes or clever positioning; it is about whether the party brought the right tools and whether they can survive while applying them. The party that arrives with a +3 adamantine maul has a fight. The party that arrives with a wand of fireballs has a problem.

Open with Fiery Bolt at 120 ft. range. 36 average Fire damage on a +10 attack against a single target shifts the encounter immediately, because the party's first instinct will be to close the distance, and closing means walking into Bladed Arm range with 10 ft. reach for two attacks averaging 30 each (20 slashing plus 10 fire). Save Poison Breath for the round the party clusters, ideally when at least three PCs are inside the 60 ft. cone. DC 18 Con and 55 average Poison is a near-kill on most casters even on a successful save.

Magic Resistance plus Immutable Form plus immunity to Fire, Poison, and Psychic means the encounter pre-screens spell choices. Polymorph fails. Hold Monster fails. Disintegrate at least lands a save with disadvantage. Banishment's not happening. The party caster has to switch to force, radiant, or thunder, and even then the golem rolls saves at advantage. Make sure the party knows what's not working by round two, because watching a wizard waste four turns on the wrong spell is not fun for anyone at the table.

The retreat trigger is whatever bound the golem in the first place. It does not flee, it does not parley, it does not stop. The fight ends when the golem hits 0 HP or when the party leaves the room. If the room has a door the party can close behind them, the golem will not pursue past whatever circle of command its creator set.

Telegraph the immunities. Smoke pours off the floor where the golem stands and never burns. A failed Fireball wreathes the iron in flame and the golem visibly grows brighter. Players who see the rules learn the rules.