Mephisto took him long ago and imprisoned him in hell.

Turn an important angel against his own kind is sweet in the demons' eyes. Izual's betrayal let the prime evils knew how to corrupt the soulstone and use them to possess humans. Inarius was once an Archangel, so he knew the council a lot, along with the knowledge of the Sanctuary, the World Stone and those hybrids. Turn him against the angels would be a good idea for the burning hell. Because of that, it's wise for the angels to take him back when they have the chance. Has there been any story about him in D3?