Someone's forgetting about a certain Demon Primarch with a name carved into his rotten heart by Dr. Draigo.
Draigo's phenomenal feats in the Warp could only be achieved in the Warp though. Since the Immaterium feeds off and is affected by emotions, devotion and belief, Draigo with his specialised training, powerful psychic presence, and belief in the duty of the Grey Knights / Emperor - alters the very Warp itself around him. That's why Daemon's who choose to confront him inevitably loose because their resolve is less than that of Draigos. However, after Draigo passes through an area of the Warp, as is it's nature of constant change, no lasting damage is ever done - reverting back to what's relatively normality for that area.
His abilities would actually be more grounded once he returns to the Matterium, since the very environment of Real Space isnt directly affected by belief and emotion, but the traditional laws of physics etc.
Have you read Mortarion's Heart? It describes in great detail of what actually transpired during that epic battle, and it portrays a very legitimate confrontation between the two:
1. Young GM Draigo was the best swordsman of the GKs at the time.
2. Draigo retrieves the Titan Sword that was left on the battlefield when the previous SGM was slain.
3. Mortarion mocking Draigo, the GKs and even Janus, the newly appointed SGM discovers his opponent has layer upon layer of psychic defences against the hated witches!
4. Draigo rather, drew upon his psychic power and enhances himself, while also being amped by his GK
Librarian (think Sanctuary + Might of Heroes).
5. Engaged in melee combat, in just barely deflecting / dodging Mortarion's killing strikes, Draigo's psychic reserves were being exhausted (his precog. was being overwhelmed in just keeping up).
6. Mortarion's daemonic talon feet inevitable pins down Draigo into dirt as all the Nurgle daemons around them cheer!
7. Mortarion's wings build up a swirling Vortex of Miasma around both of them (the same attack that killed the previous SGM and his Paladin retinue).
8. As Mortarion is gripping Draigo's helmet to rip it off and expose him to the Miasma -
9. Draigo uses his last ounce of psychic power to set Mortarion's robes a light with psychic flame - which quickly engulfs the Primarch's entire body due to the swirling Vortex.
10. Mortarion still gripping Draigo's helmet, is enraged and curses the cunningness of this "GK
11. During the Daemon Primarch's anger, Draigo realises a moment where Mortarion's psychic defences are down.
12. In that brief moment Draigo psychically and silently slips Mortartion's True Name
into the Daemon Primarch's mind.
13. Violently, Mortarion trembles, and then erupts from the inside shattering his wings, body and armour to pieces - disintegrating all the Daemons around them as well!
14. As the wheezing Daemon Primarch is found broken bodied in the dirt...slowly dissolving back into the warp, Draigo approaches and gazes upon the exploded chest cavity with the erratic beating of a black heart inside, Titan Sword in hand...
So yeah, while Draigo was fighting valiantly, he was clearly loosing the battle, and only had the slimmest chance of victory because he was passed down Mortarion's True Name (a very closely guarded GK
secret). Actually, while in choosing Draigo to be the new SGM, and thereby claiming the Titan Sword and the secret of the True Name (as only the SGM was to ever posses the knowledge), he admitted to the other GMs as he prepared for battle, they would likely need to chose another SGM before the day was done...
Edited by Waking Dreamer, 09 February 2018 - 07:57 AM.