Okay.
The Doom Lord's sword Diabolus, not only tears the enemies' souls asunder in a single strike, but (what I think) it also implants part of the Doom Lord's soul in that target. So the more the target is struck and hit by Diabolus, the more the Doom Lord's soul is attached to it.
One unfortunate warrior encountered the Doom Lord hundreds of years ago. He had the strength of a thousand men, and was given the title of a Legendary Warrior. That warrior wield one of the most famous swords ever forged: The nameless, crystalline sword that was believed to have the spirit of the gods.
Their fight lasted for three days and three nights. But alas, the immortal body of the Doom Lord was deathless and was too much for the legendary warrior to handle. The Doom Lord killed the warrior, piercing his heart with his doom blade. Before the Doom Lord could entirely kill the warrior, the warrior grabbed his sword, held his breath, and wounded the Doom Lord's supposedly impenetrable armor. That was the first and only time he was wounded.
By this time, the warrior has passed away, and the Doom Lord was the victor. Before he continued his endless journey, he tried to take the legendary sword for himself. But something was inside that sword that prevented the Doom Lord from touching it, and he also heard in his corrupted mind the sound of an Angel's voice saying: whosoever wields this sword, should he be worthy, shall posses the power of the gods.
So, the part of the Doom Lord's soul rests inside the body of the legendary warrior, and he revived that warrior as an emotionless, dull creature, with the strength of a hulk. He erased all of the warrior's memories, and then sent him to the Nether realm, calling forth the warrior when he fancies.
I don't write stories very much either, but at the moment this is all I could come up with.