D&D, by conventions, presents encounters in a "Red Queen" fashion; the encounters always just keep up with the heroes' power level. For a "compelling" change of pace, try:
- An unstoppable monster, like the Balrog/Balor released by delving dwarves. The heroes can only hope to escape it -- and, perhaps, rescue some hapless allies.
- A true horde of monsters. For all the talk of wading through hordes of Goblins and Orcs, when was the last time your party fought more than a dozen of anything at once? Try 50 Goblins.