[@RavenTLark]

Well, his friend Tsukune's soul lived inside him, and when Tsukune sacrificed himself to save Mithias' life, he basically took all the powers he shared with him into oblivion. 

Mithias had been trying to kill Alucard, to stop him from killing Bob and the rest of everything in the universes. Mithias joined bodies with Alucard as they were fighting and held him in place long enough for Bob to do a death-attack. That's when Mithias' Tsukune, and also Alucard's Tsukune (they both had one), sacrificed themselves. Loosing their Tsukunes weakened them both, and Bob killed Alucard.