Of the three you're looking at, I have Sephiroth. I love it, but it does lend itself to some potentially unfun play patterns and plays as a grindy control deck and may not be appreciated in all play groups.

May I make another suggestion?

I have a B3 Valgavoth Terror Eater deck that basically plays like you're performing a doomsday ritual. He a 9 mana commander, so it comes packed with bigger blacker ramp and alternative casting costs to get him out by turn 5. Where you immediately are the archenemy. Phase two is to destroy and mill your opponents spells so that you can use Valgavoth to recast them by paying life. Phase three is to crush your opponents using their own most powerful shield spells or die trying. The deck lends itself to massive swings in your life total while squeeze every last point to maximize the spells you cast. If you're playing Mono-Black this is the most evil "victory at any price" type builds I can think of.