Escape from Undermountain (EfU) is a Neverwinter Nights module based on the tales about Durnan's inn in Waterdeep and the well leading down to Undermountain within it.
EfU is a high-magic server without too much emphasis on roleplaying. It can be very challenging for non-epic PCs without some of the epic magic items found on the lower levels.
Access to the module is open to all, but requires a large hak pak with a vast amount of custom content, plus CEP. See this forum post for more information.