'Because he succeeded once, he will succeed again.'
It was a simple lie that Noxus instilled within Simera. By showing him his ability to sever himself, he managed to trick Simera into thinking that her sword was no longer useful.
Seeing how Noxus evaded a cut made her panic.
But obviously, this wasn't true.
There were still many reasons why she had to cut.
The slash of Simera harness the remaining light she had absorbed from outside and light became the knife that cut the darkness.
The darkness was indeed the 'separation' between two connected things. But it wasn't severance. It was merely the distance that separated two objects.
While Noxus could perform the cutting trick in his body, he had become' one' with darkness in this dark region where he manifested his evil and darkness.
And that means he could be severed!
The divine power of True Cut created a dividing rift that sliced through the darkness.
