I've got no problem with them swaggering really, the issue for me when I think of the narrative of the campaign is that these characters need to be tested by fire as it were, if they are really going to take on the Twisted Rune, which is no small task. I think Ravenloft is a great setting, and after months in the desert I think they would like a little change of scenery. If they hate it, we'll head back to FR, easy enough.
I guess my first post made me sound like an







DM, however, my prime concern every game session is that everyone at the table has fun, and so far, everyone has.
I'm glad you see that your original statements did sound a bit extreme.
But let's break this new statement down a bit.
taking on the Twisted Rune is no small task. Check.
characters needed to be tested by fire. Check.
months in desert, need a change in scenery. Check.
Ravenloft isn't a change of scenery, it's a total shift in campaign focus.
Furthermore, it's designed to NOT let PCs go. Therefore, letting them go (which their first instinct should be "how do we escape") is going to look forced.
Nextly, assuming your PCs are hot on the trail of this Twisted Rune, why are you trying to send them into something completely unrelated? It's bad storytelling, bad pacing, and disruptive to game momentum. I'm not saying you couldn't find a way to make Ravenloft tied in to this Twisted Rune (I have no clue what it is, but I assume it's a BBEG), but why take chances?
Let your PCs pick the goal and direction, and you determine the obstacles and complications to that goal. Putting some random dungeon crawl in the middle of all that is just wasting their time and inflicting your whim on the players.
Also consider this "tested by fire" idea. Shouldn't the quest to defeat the Twisted Rune BE the trial by fire. Shouldn't the 3 or 4 things the party has to face before they are able to confront the BBEG be the trial?
What you will get out of Ravenloft is Lost, Dead, or Screwed up characters that will then pretty much cancel the quest for the Twisted Rune.
Don't change the direction of the campaign, unless you truly want to change the direction of the campaign. Be especially mindful of doing so when the players are actively pursuing goals in the game.