The Vex Axis Overmind at the heart of Mercury. After killing it, it should lead to a sparrow run to the surface before the planet shatters from the explosion. The Cabal Supreme Commander and/or Champion that comes to challenge Oryx, but we kill him before he gets to our system, so he challenges us. Fallen Archon and Kell from the House of Kings. They've were watching Skolas on Venus and steal the technology Skolas obtained from the Vault.