Sorin pursued Nahiri through the broken structures of Markov Manor while Emrakul loomed over his world. Their fight was personal as well as planetary: Zendikar's abandoned prison, the Helvault and Avacyn's loss shaped what each wanted to take from the other.
When Sorin bit her, Nahiri tightened stone around him. Its continual injuries prevented the concentration he needed to planeswalk away. She turned him toward Innistrad's approaching disaster and departed.
Olivia regarded her own bargain as fulfilled and refused his plea for release. Winning this confrontation did not mean Nahiri had saved a world or Olivia could control its future. Sorin remained alive, immobilised and forced to watch.