Lots of choices here, both as a DM and as a player. I'll focus on one from awhile back because it still comes up in conversation sometimes.
We're playing AD&D 2E. This was a mid-level party with everyone in the 5-9 range and about 6-8 characters in any particular session. We were traveling cross-country somewhere in the Forgotten Realms, I don't remember the region but it was an area of open plains probably near the Sword Coast.
Camp for the night, Someone casts Leomund's Secure Shelter as was our routine. We eat, sit by fire, then sleep. LSS is a great spell for travelling (4th lvl wiz) in that it pops in a stone building with wizard locked doors and alarms and an unseen servant all built-in - no lousy tents & bedrolls for us! Also it meant no need for watches as we felt pretty secure.
So this particular night everything seems normal until the building disappears from around us in the middle of the night. So we are a)surprised b) unarmored and unarmed c) on our butts in heavy darkness. Then the breath weapon hits. I'm pretty sure one character died in that first blast with a failed saving throw.We start scrambling around for gear and get hit again. Then the thing lands and tears into one of the unarmored fighters and takes care of him. A Great Red Wyrm has stumbled across us, dispelled our Secure Shelter, and attacked with surprise.
It was as one-sided a battle as I have ever been in with characters past starting level. I don;t know that we could have taken it if we had prepared for it and surprised it - we had pretty much no chance given the way it began. I was running one of my clerics, Brutallus Maximus the First I believe, and he died of course - unarmored war clerics don't last long once the unarmored fighters go down. I know our bard turned invisible and hid - and survived - but that was kind of his thing. The only other character that lived was our paladin, who the dragon picked up in one claw after beating him down into almost no hit points, and who started bargaining for his life with one of the mightier incarnations of chaotic evil on the planet! I was so disappointed, though it was a little funny to watch the up-til-now lawful good character completely ignore any "heroic last stand" thoughts to try and save his own skin as the beast flew off with him, leaving our shattered party behind.
I believe that was the end of that campaign in that form. Even though the bard could have some of us raised I think we called it as a group, and then created a new party, recruited by the bard for revenge on the dragon that killed his comrades. We never did get that revenge as we got sidetracked with other adventures, but it was a sore spot for a long time:
- "Where did he come from?"
- "Wouldn't there be some kind of local legend about the huge ancient red dragon that lives around here?"
- "How did he see our little cottage at night from way up there?" (OK I thought this one was a little weak but it did come up)
- "How have we not heard anything about this thing if it's out flying around and attacking things?"
Anyway, we survived as players, if not as characters, and moved on. Turned out it was a completely random encounter, a good (!) roll on the wandering monster tables he had for the area. That really didn't make us feel any better when we found out, and it's still a legendary story among my long-time players today.