The oath sworn that might explain his unusual status:

By blade and blood, by shadow and starlight, I do bind my very soul in sacred Thalui - an oath that shall endure beyond the turning of ages and the crumbling of empires.
Though I am changed, though I am no longer what I once was, I shall not surrender to despair nor become a servant of destruction.
I swear by the Asuryan the protector, Khaine the warrior and Kurnous the hunter, by the winds of magic that flow through all things, and by my own immortal essence, to bring eternal vengeance upon all those who would seek to unmake this world - be they daemon, mortal, or god.
Let no force of Chaos, no servant of the Dark Gods, no harbinger of the End Times find sanctuary from my wrath.
I shall be the eternal watcher, the undying guardian against oblivion itself.
This oath I carve into my very soul, deeper than bone, stronger than steel.
Let any who would break this Thalui face not merely death, but the complete unraveling of their being across all planes of existence.
So it is sworn.
So it shall be.
Until the world's ending and beyond.