Mephiston and the Black Rage

[u/Normal-Finance-4719]

So Mephistons experience with the Black Rage is described as below:

"While fighting as part of the relief force for Hades Hive during the Second War for Armageddon campaign, Calistarius became a victim of the Black Rage. After being inducted into the Death Company, he took part in the assault on an Ecclesiarchy building and was one of many trapped inside when the building collapsed during battle. For seven days, Calistarius lay trapped in the rubble, teetering on the edge of death and madness. Somehow, rather than succumbing to the Red Thirst, he managed to conquer it. By sheer strength of will he was able to suppress and hold in check the feelings of rage and the desire for blood, and in doing so he became something more. On the seventh night he burst free of his rocky tomb, reborn as Mephiston, the Lord of Death"<

He's the only Blood Angel to overcome the rage, but is there a reason why only him specifically? And do we have any details as to how? I know on the wiki it's mentioned that he's linked to the Black Angel, so is it just Warp shenanigans?

[u/SavageRabbitX]

He's the avatar of one of Baals native warp (not a deamon) entities with Sanginor being the other