What happens at the end of assassins creed brotherhood

Ezio throws Cesare off the wall of a castle and hides the Apple in the Vault beneath the Colliseum, where Desmond and the team go to find it. When they unlock the vault with the password "72" Desmond picks up the Apple and it then takes control of him, forcing him to stab Lucy with his hidden blade. Desmond, still in a state of shock is put back into the Animus.