ha! I just now had to google it....
Powers of X #4 gave us a brief history of Krakoa - it was originally one island, Okkara, that was split in two by an invasion from what appeared to be Limbo, a realm of demons. The demons were repelled by Apocalypse and his first horsemen, and Krakoa was left on Earth, while Arrako and possibly the first horsemen were sealed away in another realm, with the first horsemen standing guard in case of another invasion