I've got a slightly different take to the previous suggestions.
Basic - Blast an enemy with lightning, applying shock when damaging force-users, or on a critical hit against non-force-users.
Special - Drain the life out of a target, healing Nihilus for 50% of his maximum health. This attack has no cooldown, but can only be used 5 times per-match.
Special 2 - Deal AOE damage to the entire enemy team. This attack drains an additional 10% maximum-health damage to force-users, spreading the damage to Nihilus' team as a heal. (Zeta) Applies Fear debuff for 2 turns, which reverses any TM gains into losses (only if the gains are sourced from abilities), also acts as a slow.
Passive - Nihilus' loses 30% maximum health at the end of his turns. This effect cannot kill him.
Leader - Sith allies are healed for 10% of their health each time they use a damaging ability. Additionally, they gain 10% offence for each enemy force-user, increased to 20% if the force-user is a Jedi. If an enemy is killed, all Sith allies gain offence-up, plus 30% turn-meter if the enemy is a Jedi.
I don't really see Nihilus as someone who particularly cares what he's draining, as long as he's draining it. Jedi, Sith, unaffiliated, etc. They all taste the same, so what does it really matter?
Also, I feel like Nihilus should have to constantly keep an eye on his health. His basic is a much more powerful ability than his drain, but he has to use his drain occasionally because otherwise he's just going to be entirely reliant on his protection to keep him alive, which isn't going to hold up if he gets focused.
Finally, I decided that giving him a unique Fear debuff would be very fitting to the character. He's one of the most terrifying Sith of all time, so he's going to cause a lot of panic and disorder in the enemy ranks when he unleashes his power like he does in his second special. This also helps to counter a lot of the current meta teams, like Zader, EP, etc, who gain a lot of bonus TM from abilities.
