Thanks guys. My game is tomorrow and I'm still stressing about it a bit (mostly because it's with people I've never played with before), but your advice has been very helpful.
I decided to go with a single danger, with plans for eventually turning this into a full-fledged Front. Possibly even as a "published" adventure. As suggested, I'm going to start the session mid-way though; either in the middle of confronting the Ice Witch as she's casting the Lay of Eternal Ice, or trekking through the yeti-infested ice tunnels that lead to the lower depth of the spire.
Adventure Front: The Lay of Eternal Ice
Danger: Hordes of the Frozen Devourer
Type: Hordes - Humanoid Vermin (the hordes are actually demons, but the impulse fits what they'll do)
Impulse: To breed, to multiply and consume
Grim Portents:- The binding magics are disrupted
- The portal begins to vomits forth an endless torrent of icy horrors
- The tunnels beneath the spire become impassable demon haunts
- The spire is entirely overrun
- The Devourer's Hordes spill forth, claiming the area around the spire
Impending Doom: Rampant Chaos
This may be a dangerous assumption, but I figure that the characters will attempt to stop the Ice Witch's spell; either through capturing or killing her, or by somehow countering the spell. Rather than solve things, that just makes them worse as the demons she had a tenuous control over go rampant when the spell ends. Hence the first and second Grim Portents. To give the characters a "dungeon" to explore, the third Grim Portent cuts off the shortcut they used to get to the Ice Witch. The tunnels lead right to the bottom of the spire; closing them off (or making them really dangerous) means they'll have to ascend the spire to escape. The fourth Grim Portent gives the demons full control of the dungeon, giving me an excuse to throw demons at the characters as they try to escape and hopefully giving them a sense of urgency. The last one has the demons boiling out into the surrounding area, causing all sorts of nastiness.
On the one hand, this setup feels somewhat railroady to me. I'm expecting to drive the characters forth along a preset path. We all know players do what you expect them to do exactly 0% of the time though. :) But on the other hand, the Grim Portents are the logical sequence of events that would happen if the PCs weren't present; demons come out of the portal, take over the spire, and then start to spread.
Well, we'll see how things go tomorrow. Maybe I'm over-thinking it.