Nether Dungeons are a cool feature added with Nether X 1.3. There are a few

Nether Dungeon

varieties, but the most common one being similar to the ones in the overworld. They will usually be guarded by wither skeletons meaning that they will be tricky, but will also be a way to get wither skulls without having to find a nether fortress. They are made of chiseled netherack, which is craftable, charred chiseled netherrack and nether columns, which are exclusively found in the dungeons and not craftable (yet).