Skeletor was a former pupil of Hordak. When Hordak kidnapped Princess Adora, Skeletor ratted him out and told the location of the secret Horde base on Eternia, which was Snake Mountain at the time. When the Sorceress and Man-At-Arms went there to get Princess Adora, Hordak fled to another world through a portal. Skeletor took over Snake Mountain and made it his place of resident and center of operations.

With his old mentor out of the way, Skeletor plotted to take over Eternia for himself; his focus, of course being no other than Castle Grayskull. Skeletor recruited many minions to help in his conquest of Eternia.

from: http://castlegrayskull.org/ (less info)