Nightingale is unable to make the decision, but Rook can tell what he wants to tell him. It’s his time to make a sacrifice. He makes the decision for her, so that she doesn’t have to feel guilty about his life.
Suri appears to try and convince him to stop, furious with grief. But she can’t talk him out of it. Rook gives the pendant to Nightingale, asking Suri to take care of everyone when he’s gone.
Suri rages against the Shadow Lord for a moment, ineffectively, before vanishing.
The Shadow Lord is appeased by the grief and pain they’ve felt, and kills Rook right in front of him. It accepts the payment, and offers them Justice.