Imagine that 3rd pic being a strike. The Vex, being the Vex, understand that since Rasputin is now awake, the threat of orbital strikes are now plausible. So they build a mind that emits a barrier around the citadel that a strike team has to go in and eliminate. But this is no ordinary strike, after defeating the boss, you now have to escape before Rasputin let's loose the fires of hell (he's a loose cannon still). *Cue epic escape as everything is obliterated around you.