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.
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.
To spawn the Black Dragon Egg you need to destroy the 6 monster spawners.