I can send my 42nd level Fighter to kick their ass?
hmmm challenges ... lets think... "ponders a lot" ... I'll better use my Baltazhar machine...

1. Healer suddenly drops - it looks like somebody is transferring their damage to him without his accept and he is fading fast
2. The Assasins past has come to haunt him - the child of a former victim has killed his/her way through the guild and found out who killed his/her father
3. A mad druid has taken over a town, implanting biological virusses into people, now they have to find a way to remove the virus/entity without killing the host whilst battling the very nature around them.
4. A ley line has suddenly changed course (or appeared if you don't use those (I didn't)) making all kind of magical dissasters destroy people and the city it self, is it the work of an evil wizard?
5. A beautiful lady steps out of the sea... oh.. that was a commercial... *lol*... but lets go with that anyway... Her charms seduces and dominates people around her as she have them working on pulling down a huge damm. The whole valley below are at danger.
6. (This requires some preperation) One of the players is exchanged by a dobbleganger, who works to get the whole group to do some evil bidding. The player himself is held captive since the dobbleganger requires him alive to pull on his memories...
7. An evil demigod has found a way to pull energy from human slaves - the group has to find his lair and release the prisoners from their magical prison before he destroyes all.
8. A bard is spreading a not so positive song about the characters, turns out he is hired by a competiting mercenary group trying to get an important job from the local noble
9. A magic item sucks in a player (or two) inside it - now the players on the outside has to work with the players inside to get them back before the evil in the item devoures them
10. All of the above

And finally some famous last words: "Come on, they are only orcs" ... (he didn't know the Rolemaster combat system)