After saving Skelos Undriel from Delfran's Hideout, he will be found back in Azra's Crossing. He explains he is on the run from agents of Jagar Tharn, and that he needs a place to hide. He decides to hide in Earthtear Caverns, but needs you to clear out any and every enemy inside.
Earthtear Caverns is found by following the same path to go to Ghast's Pass and Delfran's Hideout, but then you must go the opposite direction off the road from where you would turn to get to Delfran's Hideout. Upon arriving inside, you simply have to kill every hostile creature inside. Doing so may require you to find a Shadowkey to open a Shadow Gate in order to kill the Diamond Spider Queen. Other than that, you will be fighting rats, spiders, Wormmouths, and zombies.
After finishing, return to Azra's Crossing and tell Skelos. He will thank you and the next quest will begin.