How to trigger quest: Go to Windhelm and head to the docks (located in the southeast corner of the city). Enter the East Empire Company and speak to Orthus Endario . So the very first thing to do is to steal Survaris Atheron's logbook. Leave the office and enter the Shatter-Shield office right next door.
Simply go behind the Dark Elf and take the logbook (Be careful of stealing! Saving may not be a bad idea.). Return to Orthus and after the chit-chat, leave Windhelm and move to Dawnstar. Enter the Windpeak Inn and speak to Stig Salt-Plank. Time to join the Blood Horkers so we can get more info. Brawl with Stig and then ask for the location of the Blood Horkers hideout. Go report back with Orthus and Adelaisa. When ready, speak to the mercenary at the docks and leave to Jophet's Folly.
When you're aboard, speak to Adelaisa and she asks you to kill Haldyn. Drop out the ship and follow the marker to the Sea Cave. It's just to the south, you can't miss it! Kill both mudcrabs and loot the chest, then go up the sloppy path to find a third mudcrab. In the last room there's a Blood Horker. He's kinda strong, just like all the Blood Horkers, so be careful. Loot the room then enter the towers. As you go up, you'll find two Blood Horkers, so kill 'em, loot 'em, and proceed to the next room.
There's an expert lock on the left side, but ignore it for now. There's an Orc Blood Horkers coming up in the next room. Kill him and you can either go upstairs or down. Go down, kill both Blood Horkers and loot the place. An expert locked door down here hides Japhet and his journal (an interesting read) along with a master locked chest (that, when I opened, didn't have anything THAT good in it...).
Now go upstairs to find Haldyn. He's rather easy (especially with a sneak attack), so take him out, loot him AND the place. Return to the expert lock you passed on the way here. Out here, Adelaisa will be bombing the place, so run and kill the Blood Horkers that you find. Fight your way to Adelaisa.
Tell her you're ready to leave and you'll be back at Windhelm. Speak to Orthus and you'll receive your pay. 500G (1500G for higher level characters) and the quest ends. Adelaisa will also becomes your follower, so one more for the bunch.