In my home campaign (that's totally not ripped off from James Jacob's "Shadows Under Sandpoint" game that's up on the messageboards) the PCs have started laying plans to hit Hollow Mountain in a search for starmetal weapons effective against intellect devourers. I've got some time before they head over there and I thought it would be a really cool idea to put together a cinematic setpiece encounter that would utilize enemies way more powerful than what the PCs can handle: they're going to be in the 4th-5th level range by the time they get there.
The concept is that I want the black dragon Scarhorn (who searches the ruins on the islands for treasure to bring to her hoard) to swoop in on the PCs, driving them toward Hollow Mountain. The Taiga giants above see the dragon in their territory and start throwing rocks and spears to drive it off- catching the players in the crossfire. Dodging boulders, giant spears and gouts of acid, the adventurers have to make a run for the closest rift in the mountain.
My initial thought was to make it a series of hazards playing out across a "no-man's land" between the ruins on the slopes and the mountain, but the chase mechanics seem equally appropriate. Maybe a chase/combat encounter? I expect my players to spend time either trying to fight the dragon before realizing they're overmatched or to just try to hide until it's over (which makes sense, sure, but it's not fun). Any thoughts or suggestions on the best way to make this work?
"realizing they are overmatched" is not a trait that I've seen many parties excel at.
I've seen too many TPKs end with a GM saying "you should've known better than to try that" when in some cases a party is giving the GM credit that the 'heroic' encounter designs are intended for them to engage the present enemies.
I guess what I am saying is, if you are designing something that is supposed to be a dramatic glimpse of future enemies, and the party isn't intended to fight it... don't be subtle about the fact that you don't expect them to fight it.