You find a Black Dragon Egg here. It consists of a large rectangular room with 6 Monster Spawners with a small pedestal in the middle.
The temple is completely enclosed and you need to mine some of the bricks to get in.
Location[]
Can be only be found at or under the 30 block mark on the Y Axis (deep underground).
They can be found in the Overworld and the Nether.
Dragon Egg[]
To spawn the Black Dragon Egg you need to destroy the 6 monster spawners.