Oblivion is Nero's dark gift. This writhing mass of possibly sentient darkness has the ability to overtake most anything that Nero wishes. Once in there, objects begin to dissolve and people's minds start to go. Oblivion extracts anything he wishes - be it physical or mental - and stores it somewhere in it's depths. Aside from finding yourself disappearing, one will also find themselves reliving all their most terrible memories. Very few people can escape it, and even fewer escape without a scar upon their mind.
Unsurprisingly, Nero's unable to control this power completely. He usually has to wear restrictive clothing that suppresses the power to a manageable level - though there's always a flow of wispy darkness around him that prevents anyone from seeing his body completely.
It's doubtful that he would use Oblivion unless it's a dire circumstance. If a fight ever happens, I encourage you to contact me via AIM or PMs so we can figure things out better.
Because this is a pretty hax power, I'll leave the level a character can be affected up to you.