Poor Unfortunate Souls

Does not work with 4 ppl, need all 5 people. Tried it with 4 after we couldnt find 5th.

EDIT As of today (9.0.1) it is still requires 5 people.

This achievement is pretty hard to get depending on your gear. There are a few steps to getting it and I'll list them here:
1. Set your hearth Valdisdall
2. Have your entire party go to Blackrook Hold. Once everyone is inside the instance, look in the very first room for a blue spirit candle. Click that. Your entire party has a 30 minute buff now. If you die you lose it.
3. Kill the last boss of Blackrook Hold. Again, DO NOT DIE. If anyone dies you must leave the dungeon and reset to try again (this is the hard part). The #1 killer is the boulders coming down the stairs. They will kill you super fast, so leave your entire party on the bottom of the stairs except for 1 person who will run up to stop the boulders.
4. Once you have killed the last boss of Blackrook Hold you will get a 10 minute buff that replaces your old buff. Again, if you die you lose it. Everyone needs to hearth over to Valdisall. Jump off the cliff and goblin glider over to the Maw of Souls instance.
5. Clear trash and kill the first boss. Your 10 minute buff is now replaced with a 15 minute buff.
6. Kill the trash on the ship, do not pull a lot, the timer is VERY lenient. If you're fast enough you will also get the achievement Helheim Hath No Fury.
7. Kill the 2nd boss, and this time you will get the buff "Poor Unfortunate Soul". Again, don't die.
8. Kill the trash slowly, and make sure you interrupt the champions so you don't get feared into more trash.
9. Once on the last boss, kill her tentacles to get to phase 2 and just lust and burn. Make sure you're interrupting the torrent, don't stand in the green, and don't get hit by the bellow. If you die here be ready to have your entire group reach through your computer screen to choke you. If you can kill the boss with all 5 of you having the buff that's it, achievement is yours!

https://www.youtube.com/watch?v=7Flf1cpWMs0

Completly Soloable. You do not Need 5 people anymore.
Video Guide: https://youtu.be/LKKsDQfW-9o
If a video is too much and you want the short and sweet.

Step 1: Set you Hearthstone Near Maw of Souls.
Step 2: Go to Black Rook Hold and pick up the buff
Step 3: Complete the Black Rook Hold Dungeon
Step 4: Hearthstone back to near Maw of souls
Step 5: Enter maw of Souls and let the First boss hit you at least once. (! IMPORTANT !)
Step 6: Finish the Maw of Souls Dungeon
Step 7: Enjoy Lagan.