Death Neo Guardian

Osiris was the Necromancer's protege. He monitored the Necromancer's life support systems daily, and he became intimately familiar with the Necromancer's history, as well as his ideals. Eventually, when all the Neo Guardians were created, the Necromancer used osiris and turned him into the Death Neo Guardian, using him as the final component of the core that powered the newly built body of the Darkness.

As the Death Neo Guardian, Osiris has the power to cause pain, suffering, and instant death on anything. The Necromancer realized how the emotional and reckless Osiris would be corrupted by this power, so he saved that for the end. He forced him to take the Gem, and then sealed him into the core before he could even do anything. Poor Osiris essentially died there...he lost his soul and consciousness when the Gem Guardian merged with him, since he didn't have the protective Neo Suit the other Neo Guardians had. Of course, the other Neo Guardians were absorbed into the power core as well, so it didn't help them much either.