I watched (at least part of) this movie as a kid. English language live action, likely 1990s but not later than 2000, fantasy film. I only remember one scene in detail - a group of people ride on horses through a forest when the forest floor swallows them up. I believe it was a mud/quicksand-type sucking-them-down-slowly situation, though it might have been more of a trap-gives-way fall. The group falls through into an underground cavern/chamber (many, if not all, of them survive the fall). (…It’s possible the group is not on horses when they are swallowed by the forest floor. I’m pretty sure they are riding horses at the start of the sequence, though.)
Not quite as certain about this part, but I think the survivors then encounter a being in the underground cavern (it’s possible the being is not actually in the cavern but communicates using a magic mirror or something similar?) who is now imprisoning them, with whom they may have to bargain to get out. The being may be an already-known antagonist.
This has bothered me for over twenty years so if this rings a bell for anyone I’d be very grateful (and excited to rewatch and see if it’s as scary as I remember).
