I would like to add that apparently my initial thought of Mordekaiser not having used Soul Manipulation in that specific feat is correct, as I just read the entire fight with the knight.
The Siphon Of Darkness was blocked due to "Demacian Steel" and just at the end of the fight, when the Knight was dead, did he manipulated the Knights remaining soul and made him into and underling.
I will just quote the entire fight here, since I think its way better than just quoting a part of it which could be easily misinterpreted:
The garrisoned soldiers and men-at-arms waited for him in the courtyard beyond. Weaklings all. His gaze swept over them as he sought a foe worthy of his attention. His undying gaze settled on a silver-clad knight that stepped out to meet him, sword drawn.
"Begone, revenant, or I shall see you banished," said the knight. "This hamlet and its people are under my protection."
Rising to this threat, a host of specters and translucent warriors manifested in the Black Mist behind their master.
"This one's soul is mine," Mordekaiser said, holding the eager spirits at bay. His voice was deep and sepulchral, the timbre of death itself.
Mordekaiser pointed, and a cone of malignant unlife burst towards the knight.
The knight's armor shone brightly for a second, then returned to its normal, mundane form, leaving him unharmed by Mordekaiser's necromantic sorcery.
"Demacian steel," sneered Mordekaiser. "It will not save you."
He stepped forward and brought his spiked mace down toward the knight's skull. The strike was met with a two-handed parry, though the power of it forced the knight to his knees. Mordekaiser towered over him.
The knight spun away, avoiding Nightfall as it swung toward him in a lethal arc. He sidestepped and sank his blade deep into Mordekaiser's side, biting through the banded links and chain. To a living man, it would have been a mortal blow, but to the armored behemoth, it was as nothing. Mordekaiser backhanded the knight across the side of his head, sending him reeling.
The Iron Revenant stepped in to end the fight, but the knight turned aside his strike with exquisite skill, and rammed the point of his blade into Mordekaiser's chest with all his strength and weight.
With a wrench of metal, the blade punched through the breastplate above the heart. There was no resistance from within, as if the suit were hollow.
Mordekaiser grabbed the knight by the throat in one giant hand and lifted him off the ground.
"You thought you could protect these mortals," said Mordekaiser. "But know that it will be you who slays them."
He squeezed, tightening his grip on the knight's throat. The mortal's feet kicked in the air.
Mordekaiser watched closely, eyes burning, as the life drained from the knight. Finally, he dropped the lifeless corpse to the floor.
Mordekaiser knelt and placed a hand upon the dead knight's chest. When the armored giant rose, he drew forth the shade of the dead warrior.
The spirit of the knight looked around it, horror writ in its spectral eyes.
"Now," commanded Mordekaiser, knowing that the shade was powerless to resist him. "Kill them all."
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Which is what I initially thought and goes against him having Soulsteal/manipulation on his every bread and butter moves while at the same time confirming that once someone is dead he can use their Souls to do his bidding.
It also confirms that the "his soul is mine" claim was actively meant to mean him making him an underlying after having defeated and killed the knight.