Now you're gonna head on out to talk to Harben Ashersmith of the Shining Serpent Inn. He'll make you look for Krestal and Jemanie - go to them and you'll get closer to finding out the mystery of the undead.
Jemanie will tell you about the strange house and give you a key to it and talk to you about his brother Torin (see sidequest article for Beggar's Nest). Using the key, you can now get into the strange building on your map. Its the Snake Cult hideout now. In the Snake Cult Estate, you'll be fighting off the cultists and their leader and thwarting off some kind of summoning ceremony. When you defeat them all, you'll find a journal called "In service of Gulnan". The book will talking about the Sword Coast Boys and how she's corrupted them. This will also lead you to go on to the Warrens of the Damned. (I of 2 paths)
Krestal on the other hand, will tell you about the Sword Coast Boys and the warehouse they've been sneaking in around on. Go to the warehouse and take out what's left of the Sword Coast Boys. Now in this area, you're going to meet up with Drawl. He's no longer human, yep he's a lich now! Very creepy. But you can do it, I know you can! After defeating Drawl head on out to the crypts. (This is another one of two paths)