Why had the Lion ended up in a stasis field? That story began several hundred years earlier.
Now, someone within Chaos wanted to ascend as its fifth god. Yes, the very being the Dark Mechanicum had spent ten thousand years bowing and scraping before. He sought Abaddon's cooperation in collecting a number of relic keys. Once they had them all, they could attempt to create a weapon capable of utterly destroying the Imperium.
With the Imperium destroyed, Abaddon would get his wish, and that being could ascend as the fifth Chaos God.
One of those keys happened to be hidden in The Rock, the fortress-monastery of the First Legion, the Dark Angels.
Abaddon and the envoy of that "fifth god" attacked the place. Then... well, the other four gods certainly weren't happy about someone trying to become a fifth. They sent a Daemon Prince to interfere.
The ensuing melee woke the Lion.
We still didn't know how the Lion had passed those long years. All we knew was that when he appeared this time, his hair, temples, and beard were white. He looked old and worn, but his fighting prowess remained undiminished.
From then on, the awakened Lion drove out the enemy and saved humanity from peril... hang on, that script sounded familiar.
After some of this and that, and that and this, with Guilliman, the Lion led his forces on campaign.
As the fighting continued, he pushed into territory belonging to a force of Necrons. As everyone knew, the Necrons only began awakening en masse in 41K. Before then, they had been sleeping underground. Back in the Lion's lifetime in 30K, that region had been part of the Emperor's Holy Domain, Imperial territory.
The Lion hadn't been awake for long before his competitive streak resurfaced.
*After Guilliman woke up,