@Trexxe It's explained better in the novel this manga is based off of; his power is to corrupt the holy power within them by merging his essence as the Demon Lord of [Evil] with other creatures and in doing so demonifies the children so spawned. You can't demonify a demon, so doing it with his kids only increases his numbers (which he did, enthusiastically), not their strengths. Basically, the power boost comes from the corruption process transforming the base model into a demonic creature variant, not from just spawning children in general. It is specifically outlined in Chapter 9, Volume 3, if you want to read it.
@DatOneGuy We don't know to what extent this "evil" power necessarily has to be evil themselves and do evil things. If the power is just to impregnate, then you could still consensually have sex with your demons and impregnate them without forcing them/raping them.
Well sure as hell didn't expect that. That's quite the turn. I'm really reading the series after a long time and I completely forgot everything so that was the second time this has surprised me.
Yangire Erofu. <3
She was already better than the others on the basis of being the curviest. This just pushes her further out in the lead, though strangely I suspect it might remove her chances at getting a proper name.