Undead Dragons have the appearance of a black dragon with a zombified look to its flesh and has a very skeletal appearance, (hence the name Undead Dragon) that is spawned forcibly at 4000m,

Undead Dragon

Undead Dragon Ingame

The Undead Dragon is notoriously difficult to defeat, due to its high damage output, high HP, and the fact that it only spawns in hell means zombies and other monsters will swarm the area. The Undead Dragon can demolish nearly all blocks except Bloodstone, Space Rock, and Space Goo.

They may drop Diamonds, Bloodstone, Copper, Gold and Explosive Powder