The Dark Forge

The dark forge is a mid-game structure that generates under mountain ranges, but always above Y=0. It can be located with a map from a cartographer villager.

The structure is a large deepslate-brick chamber, containing a boss - which is the forge itself. It will kind of look like a mash-up of a blast furnace and an ominous vault, though twice as big and have pipes feeding into it. It cannot move, so it summons fireballs and mini 'forge golems' (which certainly can move!).

It will drop 1-2 dark forged upgrades and up to 16 dark forged nuggets, and 4 of those combined with a crystal heart (dropped by forge golems) creates a dark forged ingot, which combined with a dark forged upgrade can upgrade iron equipment to dark forged.

3 dark forged ingots and 2 sticks in a pickaxe shape, creates a forge hammer, which can be upgraded with all sword enchantments (except sharpness) and can also have breach. It's special ability is to pin mobs down in place for a few seconds.