The Apocalypse Dragon requires the four elements Plant, Lightning, Cold and Metal. An good breeding combination is Magnetic + Lichen Dragons. It was introduced for the 'end of the world' on December 21, 2012. It's available until January 9.
"When the mythic volcano Garita erupted, wizards feared the end of the world. Luckily the only thing that emerged form those ash clouds were these incredible dragons. Be careful with teething young Apocalypse dragons as they love to gnaw on wizard footwear. You wouldn't want one to swallow your sole."