I briefly met a demon named Flambeau about two years ago. We'd taken on a fire demon and some minions for Silvana's annual Champion's Challenge and closed a gate to the elemental plane of fire in the process. Flambeau showed up and wanted to kill Tremere, who'd done the actual gate closing, but I killed him instead.
Spirit assimilation? I know that most extraplanars grow more powerful by absorbing souls (though one soul, or maybe a few, dominate the bunch) and that taking out demons sometimes amounts to making it smaller by knocking some of the souls loose. I suppose some demons could absorb spirit energy as well; all I really know is that harnessing spirits is a lot harder than harnessing souls because spirits aren't atomic and tend to seep back to whatever pool they came from. (This also makes your case interesting in that fragments of spirits aren't supposed to persist across centuries; hopefully you'll remember something about whatever deal you made for this as you recover.)
Do you remember anything else about the Grey Elf...either what sort of work you did for him or what warnings of doom he'd made? Silvana and Stealth have heard vague rumors about this elf, who is remembered as the Swordmaker, but historical knowledge often doesn't survive so well in the Forest.
Through the whilg, I've had visions involving a golden arch, but in my visions there's a woman on the arch who we think is the Shade Lord. The whilg seem to think that I'm destined to defeat her before getting killed by Rakni; this might just mean that the whilg want the Shade Lord taken care of and are hoping that their prophecy will induce me to pursue that course of action.
Several races have stories and conceptions about what it means to be a Red One, so it's probably something, but since it seems to be a Chaotic thing I'm hoping it'll end up meaning primarily what I allow it to mean. The Zzukiss conception of fighting extraplanar evil is cool, the githirat idea of following the path of truth (possibly a metaphor for good, or at least one conception of good) and fighting lies (which may be a metaphor for demons) is fine, but the dwarven concept of being born to fight a chosen foe in a combat where both parties die is not so cool. We'll see, I guess.