The fight with Lareth can be handled in a few ways. The easiest way is to just kill Lareth outright (which usually requires a critical hit on a single character run). If he gets under half HP he will talk. Engaging the fight again will have Lareth plea to Lolth for help, which summons fiendish spiders in the soldier barracks and causes they to join the fight. If you have trouble doing a quick kill on Lareth, have him join you and hit him over and over until he dies.

Being I am using the Circle of the Eight module add-on it causes an ambush outside the quick exit. However, if you bring up the map just as the talking is over, you can completely bypass this extra fight (if you also use the Circle of the Eight module). Otherwise, there is nothing outside of the cave. Either way, head back to Hommlet.

When back in Hommlet, have Burne identify Lareth's Ring of Freedom from Movement and sell off the other stuff. Talking with Burne after Lareth dies will also allow the lone cleric to move on to Nulb, one step closer to the Temple of Elemental Evil.