They were the Prince(s) of Darkness in the Underworld, who were 1000000000000000000000000000000000000000000000000000000 years old when they lost their memory. They now have to find the whatever and reclaim their whatever in the Island of Nobody Cares.
P.S. I already discussed this.
Official Crawdaunt of the Pokemon XY Boards and Team Miror Infinity "It's Faaan-daaan-gooo! Breathe the A's in." -Fandango