Guilliman never died. His soul never passed on to the warp.

‘I have seen this,’ Mortarion says, cracking a half-smile that makes the puckered skin above his rebreather flex awkwardly. ‘I believe I am the first to do so. Guilliman will revive. The numbers tell me this, and I have travelled far within the Garden to confirm it. There are groves that hiss his name in the wind. I look into pools and see his face staring at me. Guilliman! Stiff, dreary Guilliman. I’d have preferred another one. The Lion, perhaps, whom I always quite admired. But one will do, even the dull one.’

‘He… lives?’ Vorx asks uncertainly.

‘He never died. His soul was bound to his body – you could see its echo in the warp, if you knew where to look. But the day is coming, and here’s the irony – the Gate will break, and that shall release him.’

- Lords of Silence