Funny thing is, there is no Demon King. Demon's aren't a unified race; They're just a collective number of malevolent creatures which have the munchies for humans, to put it childishly. Also, even if they were a race, those are some mighty high aiming goals for a spent up Demon-turned-Half-Demon who only has a few years to live, if he uses no power at all. <.< Seriously, there's one reason only Renard can be so old; The seal. It keeps his power in check. And, after living for nearly FOUR CENTURIES, even the most powerful of Demon Lords would be at a level where one of you could take it on. Giving a human Comparison... Young Demon Lord = Young Human Male. Renard = Old man. If Renard were Human, he'd be shouting 'Gerroff mah lawn shunneh!' and waving a cane around; If the cane was light enough for him to lift. The pro is, Renard is a Demon, so he doesn't decline like that, and he has 400 years of experience fighting Demons with Zaphain (AND he was taught by Cyphus Leroe) but the cons are, he is severely weakened compared to his prime, he can't use much more than 10% of his power for very long otherwise he'd rapidly start draining himself. His most favourable role would be acting as a support character, in combat. He's powerful, yeah, so at the start he's definitely your ace... but his potential is all but run out, whereas everyone else has room to grow. I was actually hesitant at first to let the character pass, but I realised a way it COULD work, and thought of that. So really, Adwarf, if you want to pre-emptively get on the waiting list, I encourage it. Because if there is ever a boss battle where things are looking bad... and you decide to go all out, for the team? It's pretty much a Suicide Attack. The laws of nature say that Renard should be dead, many times over. And because of this, every time he uses his abilities, his life expectancy drops dramatically. It's not a matter of decades; It's a matter of years. Having that Partial Manifestation alone is shaving days, if not weeks off every few seconds he holds it.
Sorry for the assholishness, but I feel it's my duty as a DM to point this out; I set down my rules. I was nice enough to let you in, even though your character was pushing the rules, (400 years old, full Demon rather than Half, Demon LORD to boot) and I'm not going to let you keep this character for free. Again, sorry for being an asshole, but... you see my point, right?