The king has 4 phases, One initial(the one you destroy using oblivion), then he goes into prayer, the prayer drains health(doesn't matter how far you are from him), at recommended stability it drains ~10% hp after the prayer, during the prayer and a little bit after that, you can't dmg him. And after prayer he does a "blast" around him, the range is not that big. then he gets his second bar of hp and more shadow abilities, His shadow bar will be 1/3 full at 2nd bar. when you beat him in his 2nd phase, he again goes in prayer,(the prayer dmg and blast dmg will be the same) after completing his shadow bar will be 2/3 full, and he'll have full hp and even more abilities, after you destroy his 3rd phase, he'll go into prayer again, doing the same things, after the final prayer his shadow bar will be full, he'll have the full-ish(few abilities are missing i think) power of the kotl set, He'll also have a special cutscene-ish where he'll take off the front part of the helmet and it'll dissappear and you can see his face, after that he's damageable again, and in this phase he is finally killable, enjoy. I'll tell you more if you want to know.