Purple Eggs are loot sometimes dropped by Sea Serpents. There used to be a quest in which you could turn in 48 Purple Eggs to Fallon at the Restaurant in Mirith for a Ring of Lightning Bolt and 1000 experience points. However, in the May 2013 update, this quest was removed , then brought back later on. Purple Eggs can be eaten raw, but because of their value it isn't recommended and just wastes gold.