We really don't know enough about it to speculate what it's actually like. The book reccomends just using Hellfire with the serial numbers filed off, and that's what I'm doing.
That said, your version works fine mechanically, and the only change I would make is dropping the bonus to +1 and, in exchange, giving it the same ability as Soulfire to reduce Toughness abilities by one level. That seems very 'corrosive to the fabric of existence' and thus very appropriate.