( Lucifer's POV )
Lucifer was thoroughly enjoying his time fighting Regus Aurelius as he got to explore a lot of previously untested powers of the seed of darkness when he faced the vampire monarch.
For one, he found out that whenever he bruised or nicked Regus, the darkness element coating his sword caused a decay effect in the nearby skin of where Regus was nicked.
As Regus was a monarch with robust vitality, the damage was minimal and only spread to about half a centimetre across the cut, but it was still an exciting discovery for the devil as it meant that when he tapped into even more of the seed of darknesses powers he would be able to increase this effect significantly.
A second ability that he noticed was that he no longer felt recoil at all when his sword met against Regus's.
