After looking at the relevant scripts it looks like killing Fhjull will activate the portal back to Curst, but only if you've already talked to the Pillar. If you kill Fhjull before you KNOW about the pillar, it's game over.

If you killed him after finding about about the Pillar but before talking to it, try re-entering his "house" and it might open up the portal. It calls a Die() function rather than GetDead() so it's possible that you're out of luck. I'm not sure if Die() only gets run once right when the creature dies.

Heh! Thanks for going as far as going through the game scripts to answer my question! Hardcore!

Anyway, like you described, you'd expect the portal to appear if he's dead (or at least instantly game over or something to let you know that it's pointless to continue) but it hasn't appeared for me in the location given in the walktroughs i've looked in, even after entering / existing his house.

The things is... It wasn't me who killed him, but that host of demons that appeared since I gave the pillar of skulls his location, which feels like something the game should be able handle. It did trigger his final dialogue just before that battle, but I suppose I didn't think to ask him how to get out of there before ending it. That's what I get for keeping on playing when I'm really too tired I guess... Then the battle started, whereupon he ran into a corner and just disappeared. Is that's what supposed to happen? Might be a glitch that he didn't really die there?

Well if he'd actually died, it would have triggered the portal key, so it's likely some weird bug and he vanished rather than being killed. I've had a few instances of NPCs walking off and getting stuck on the edges of the map, instead of being where they should be. It could be that also. I guess just reload, and save in multiple slots next time... =\ Also, you can successfully lie to the Pillar about his location if your CHA is high enough.

As for the script thing, I use Near Infinity, which is a great editor for all of the Infinity Engine Games. It's a pretty simple thing to open everything up and take a look at it. It's a powerful editor too, but it takes a little getting used to. Unfortunately, NI can't directly modify everything in your savegame for some reason, so you couldn't just manually set the correct variable, but it would be possible to spawn a second copy of Fhjull in the savegame's area files. That might start his final dialog, or it might not. I wouldn't recommend it if you're new to modding, either. You're probably better off just reloading an older save.

It's times like these that I wish PS:T had a "cheat" function like all the other Infinity Engine games. It's pretty annoying.