Death cannot touch Kel'Thuzad so long as his phylactery remains, and in defeating the lich the Phylactery opens and empowers him. During this window, players may enter the Phylactery to destroy the last vestige of Kel'Thuzad's soul before the door closes, but while they do so the armies of Kel'Thuzad emerge from the soul stream to asault the raid.
Each time the Phylactery empowers Kel'Thuzad, he gains a stack of [Necrotic Surge]
After the Phylactery is destroyed, Kel'Thuzad unleashes all his power and minions to obliterate the raid before death can finally take him.
On Mythic difficulty, Kel'Thuzad starts with two stacks of Necrotic Surge.