GM:  Your horse moves sluggishly and its natural sense of the way  appears askew due to its state.  You see none of the scant trail-signs  that you marked in your mind on the way in.  Everything looks the same.   Over the next rise your worst fears are realized in the shape of a deep  gorge.  Your horses in their best state could leap across the  chasm...but they are tired.  Doubling back may be the only way out.  But  your pursuit closes in on you.  <We are now down to next success or  failure dictates the outcome of the Skill Challenge>
D:  This place looks familiar from stories I was told in my  youth.  Many thousands of years ago, wildmen once roamed here and used  hot springs for winter baths.  A story told of a barbarian king who won  this territory when the ground opened up and swallowed his enemies.  As  our enemies close in for the kill and my friends dismount their horses  to prepare to fight, I consult the earth spirits and the annals of my  mind for a means of earth-borne egress.  < History - Failure>  
GM:  Were the stories right?  Perhaps yes.  Perhaps no.   Regardless, "earth-borne egress is what you get."  As the gathered force  of the snake-men surrounds you, the earth yaws wildly beneath your  feat.  Screams from man and beast alike fill the air.  The smell of  dank, dusty death fills your nostrils for a moment and the splash of  freezing water sends you to unconsciousness. 
<Skill Challenge to escape with the idol lost.  Complication ensues.  Sinkhole manifests.  Good guys lose  2 HSes apiece.  "Mole-man" pilfers idol and butchers horse while PCs are momentarily unconscious and brings the horse-meat and idol back to their lair.  "The Descent" style Skill challenge to track the mole-man, recover the lost idol, and escape the Underdark ensues.>