Necropolis

A large structure that is a little larger than a woodland mansion. Composed of mainly cobblestone, mossy cobblestone, and bone blocks littering the floors. The terrain would be uneven to show how degraded the place is. This structure spawns deep underground, around where deepslate overtakes regular stone. There would coffins made of wood, some open some closed. The closed ones have a chance to have a skull inside if you break the wood in front. There would be halls with waterlogged slabs to make it seem as if water had at some point flooded the place. These halls connect broken rooms that have mob spawners, chests, and sealed tombs. Snuffed out candles will litter the place to show that people used to come here to mourn the dead. A lot the bone blocks are infested with silverfish. Other than silverfish, only undead mobs can spawn here, and all undead mobs CAN spawn here. Zombies, drowned, skeletons, strays, husks, wither skeletons, you name it. There should also be a new undead mob or two that spawn exclusively here like a lich or a necromancer.

For loot, maybe chests can have a new spooky music disc. Maybe a comical one with xylophones to mimic bones playing a tune, or a new unnerving song the likes of discs 11, 13, and 5. 

Being all about the undead, it would give players a reason to make a smite sword.