We don't know yet. My current theory and the most popular one is that his magic is either brain or soul magic, continuing the theme with the Zogratis family, body, bone and blood. It could be mad hax tbh. Hopefully we'll see it soon.
Imo Astaroth+Lucius>Lucifero>=Astaroth>Lucius with out any devil powers, just on his own. Unless we find out he's extra powerful lmao.
@Seth @Gol D. Roger @Shadowlord123 your thoughts on this?
Hard to say. Lucifero has definitely been portrayed as the strongest demon, yet we've also been told multiple times that the Underworld has three Rulers. Based on this, Lucifero should be above them, but not by an overwhelming amount. The other two should be capable of somewhat competing with him and threaten him, otherwise there'd only be one Ruler the way I see it.
We also don't know if Lucius really has Astaroth's full power or not. I assume he has it, but the truth is that we don't really know what kind of relationship these two have, nor what Astaroth's current status is.
The fact that he choose to wait until this point to do what he did strongly implies either one of these two things IMO:
A. He couldn't have defeated Full Power Lucifero on his own and had to wait for him to be considerably nerfed (half of his power taken away and presumably already absorbed by him).
B. He might have been capable of defeating him, but he wasn't sure at all of suceeding. AKA, they were on very similar levels beforehand and couldn't afford to take any major risk for his plan.
What's clear to me is that the current events HEAVILY tipped the scales in his favour. Not only Lucifero lost half of his power, but Lucius presumably had already consumed one of his hearts (and half of his power?) before facing him on the 7th Level of the Underworld. Lucius presumably became significantly stronger, while Lucifero became much weaker.
I think best case scenario for Lucius, they were on similar levels before these events, but him being beneath Full Power Lucifero is also very possible.
We kinda don't know how strong Lucius could've been on his own prior to having any devil power, but it's very likely he was insignificant to any of the Three Rulers of the Underworld. That is, his own strength added to Astaroth's wouldn't have made much of a difference compared to the strength Astaroth had on his own (assuming Lucius has Astaroth's full power), most likely.