Liliana commanded the Eternals under a contract that gave Bolas terrible leverage over her. Obedience kept her alive while forcing her to serve the slaughter. At the decisive moment, she stopped choosing survival. She ordered the Dreadhorde and the God-Eternals to turn against their master.
The contract answered her defiance by destroying her. Gideon intervened, extending his protective magic to Liliana and taking the punishment onto himself. His invulnerability could not save him from a death written into someone else's bargain. He died so that Liliana could continue the revolt she had finally chosen.
With Bontu and the redirected army attacking Bolas, the structure of his ascension collapsed. The victory belonged to many plans and many fighters, but its emotional centre is this exchange: Liliana accepted the cost of resisting, and Gideon refused to let her pay it alone.