I Got Your Endgame Right Here, Buddy

Tower Golem

These enormous magical stone constructs--allegedly a hybrid of goblin alchemy and dwarvish engineering--are as tall as castles and feared in every civilized nation. In siege warfare, the looming juggernauts are employed to crush battlements, stomp infantry, carry archers in their stony crowns, and act as living bridges across moats and parapets.

In order to control a tower golem, one must--during the battle--employ a specially-prepared and highly-accurate scale model of the battlefield upon which it is to be deployed, including a model of the golem itself quarried from the same stone as the golem.

The golem's hulking bodies burn magic quickly, and few remain animated more than four or five hours. When the energies that power them dissipate, they slump over, immobile, and are often converted into fortified residences. Occasionally, depending on where they come to rest, their "corpses" become integrated into the strongholds they once were employed to destroy.

On rare occasions, it has proven possible for clever wizards to re-animate these living siege engines after their "retirement", and on rarer occasions, the creatures have been known to live on in strange and subtle ways long after their original purpose has been served.

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Crunch:

A tower golem isn't something ordinary PCs should be able to take on directly. Big h.p., big damage, big in general, great A.C.--4-10 storeys tall. They're slow, though, so their to-hit on any individual target smaller than an elephant should be pretty hopeless. If you wanna really get under the hood you might say it ignores any AC due to armor but AC due to dexterity or size counts double.

In most cases, you'd be fighting a tower golem in a large-scale-battle. Taking it out would be an objective in a daisy-chain-of-death-and-destruction-type-situation, by hitting it with a catapult or tricking it into a trench or taking over the controlling model or by going inside the golem and attacking its power source or vital systems or something like that.

High-level PCs, of course, might have a shot at it, in which case I'd say remember that it's big and slow and made of stone and its vulnerabilities should reflect that--picks do more damage than hammers which do more damage than axes which do more damage than swords which do more damage than arrows. Fire likely only makes it angry.

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Sketches by me. They're a little more cartoony than I imagine the thing to be. Hopefully you get the idea.