can point out what says explicitly that he is gone for good? Its like Gaius, until we have hard evidence saying he's dead...
Wait...what if that's who put on that mask at the end of 3.2...where've the darksteelfoil hats gone to?
But, my problem with nothing explicitly saying he's gone for good (journal says "executed", but I can't take that to be hard evidence that he was entirely consumed, just his "mortal" form was killed. If he was, I'd imagine the Eye would have been completely drained as well, but it wasn't. Far from it, really...), just was used as part of an aether battery, is that if any bit of him "survived" within the Eye, he could return. He has the Echo, he *is* immortal; you'd have to be completely sure you destroyed him to be sure he's gone.