Izh'kafel was a necromancer, like Sauron, and also was taught by Morgoth. Morgoth never considered him as a servant, which Sauron deeply hates. He is known to be Sauron's rival during the early days. He has been known since as the King of Frost.
He was once a follower of Morgoth, but only a few has known this, for he fled before the Great War happened. He established a kingdom in secret far south and none has known this. He practiced dark magic in secret and so as a result, he learned to create the Undead, which composes his whole armies. Before Morgoth was defeated, he cursed Izh'kafel, who left him in his time of need and so his kingdom was lain waste, frozen by ice, his power vanished, with Morgoth's fall. He was sealed within his tower, which was higher than the any other, but his power was not ceased, and he still have some loyal followers, which will obey him in anyway possible.
He was also aware of Sauron's fall, and plans to take the North as soon as his curse was no more. He knows that the only way for him to get out of his tower and regain power was to regain his blade, Frostmourne, which was lost to him in an unknown way. However,Sakos Flamefist has kept the blade after the finding it and has made multiple enchantments to prevent Izh'kafel from holding it.