Similar to the other solutions, make sure the difficulty is turned right down and remember to save regularly.
Speak with Sheogorath for yet another quest. He will ask you to go to Cylarne and there, you must decide to fight with the Golden Saints or the Dark Seducers.When you get there, you will find a Golden Saint standing guard on your right and a Dark Seducer on your left. I, personally, chose to help the Saints but you can help the Dark Seducers. Whichever you choose, go through their respective gate and speak to the leader of the group (Ulfri for the Seducers, Kaneh for the Saints). You are given an option to scout out the enemy positions but I just went ahead and led the charge. This guide is for the Saints, quickest in my opinion. The members will head towards the opposite gate and kill the guard there. You should follow them and go into the entrance on the opposite side to your own. I found that they managed to get caught up in their own little fights and you could just wander on through so I did just that. When you get to the Alter of Despair, you will find more enemies and the leader of the opposite group. Kill the leader and you will be told to clear out the remaining enemies. Once you have done this, your commander will then sacrifice themself in the altar and you will be directed to the main courtyard where a flame will be burning. Step into the flame to retrieve it and then fast travel back to New Sheoth. You are then given the option to light it for Mania or for Dementia. I chose Mania but it doesn't matter, you will just get a different reward which you probably won't use anyway.
Achievement number 5 unlocked.