Well, in a sense, most of his Black Crusades were successful, though not in a conventional manner. Think of it as the goalposts being in weird, non-euclidean locations.

You mean undefined? Basically how I imagine a Black Crusade goal:
"Do kill some Imperialists."

So they always can claim they succeeded But idk, Chaos needs a few bigger achievements. They got Bile, Abaddon, Typhus, etc. But they are not using their full potential, they should crush the Imperium with all their Daemon-Primarchs easily....

Anyway. That belongs elsewhere, not here ^^

I do concur, on all points. Though, just to clarify, conquest wasn't their primary goal during most if not all Black Crusades. That's why I say they succeeded for the most part. Just wanted to put that out there, for the record.

Definitely not. It would be a Legion Power Sword and would furthermore be a Daemonic Weapon with Willpower 70 and a Binding Strength 4, putting it at a decently respectable statline of:

Class: Melee, Range: -, Dam: 1d10+18, Pen: 14, Special: Best Craftsmanship, Power Field, Balanced, Daemon Weapon (Impossibly Sharp, Vicious, Piercing), Legacy Weapon (Legacy of Slaughter/Conquest)

All in all creating an absolutely horrifying weapon.