when you use a skill you get 4 mana because of the archangel staffs and you gain ability power based on your maximum mana ...+rabadon with 30% of ability power .... witch means if you stack your abilitys properly you can get up to 100ap :D.

You use revive when you die in a team fight and someone is below 30 % of hp . After you use revive you activate the ulti and you secured the kill . Clarity is used when you dont have enough mana for ulti to kill secure ! :D