Editing Imperium Asunder Campaigns (section)

===The Exodus===
At Terra, the Warmaster sneaks in, disguised as a Custodian and assassinates the Emperor. There's a few primarchs on hand, and they're on the walls in the maelstrom of battle, so Klaus goes after him while Faustus and Engerand? (or one of the other Primarchs) remain to keep the front stable, as best they can. They organize a retreat.
Faustus takes the Emperor's body and puts it in a stasis casket guarded by a special Oathsworn detachment. Probably an honor guard of Custodes, too. He loads it onto a ship. Engerand loses track because FUCK FUCK FUCK, Klaus is dead too, now.
All seems lost when the combined Scion and Sky Serpent drops out of the warp. They cover the retreat as Faustus drops the moon.
Somehow, the Casket ends up on the Scions flagship. Cue dramatic scene where Sarco looks down from his casket and can't even touch his father.
Dreadnought tears.

So the Scions and Serpents cover the retreat all the way back to Amaranth, mostly Scions since Xun speeds off to Tepectitlan.
They reach Amaranth as the storms worsen. Ships are unloaded with the treasures and relics of the lost West and hidden in the catacombs of Amaranth as a bunch of Judgement Bringers begin the Century Siege.
So the Custodes and Oathsworn stay with the Casket down in the Omega Vault, while above them the siege rages. The whole sector is under siege, really. (The other option is that the Casket is picked up by the Scions, it's guard having been slain or something.)