This is a small dimension with mobs from Minecraft dungeons. Like the end, you can't get out until killed or you kill the arch-illager.
Mobs: in this dimension you will encounter royal guards. When killed, they have a chance to drop the royal shield which makes illagers neutral in the overworld. You will also encounter enchanted vindicators and pillagers that have more health but have the same drops.
Bosses: there are 2 bosses the redstone golem and the arch-illager. You encounter the redstone golem halfway through. It drops the redstone channeller that can be used on redstone dust to make the signal last 30 blocks.the golem has 400 health and it's attacks deal a heart of damage. The arch-illager drops the mystery orb which is used a bit like a bow but deals 20 damage and ignores shields. He has 100 health but his slow moving projectiles deal 20 damage and ignore shields.
Entry: to get to the dimension you need a Portal the same as a nether portal but instead using crying obsidian. The portal only works in the end and only if you have killed the wither and the Ender dragon twice.
Appearance: the dimension is a massive obsidian castle.
Other rewards: if you clear this dimension villagers require half as many emeralds for trading.