As I have said in my other solutions, make sure to have the difficulty turned right down and to save regularly.
To unlock this final achievement, you need to complete 3 quests: Symbols of Office, The Roots of Madness and The End of Order.
When you speak with Sheogorath this time, he will end by transforming into Jyggalag and disappearing. Speak with haskill and he will tell you that you need to make a new Staff to use in the Font of Madness. Head to Knifepoint Hollow which has now been marked on your map and speak to Dyus who is behind a crystal door at the end of the corridor. He tells you that you need to find two things to make the Staff; the eye of Ciirta and a branch from the Tree of Shades. Ciirta is in the Howling Halls and the branch is in the Grove of Reflection (Milchar). It doesn't matter which order you collect the items in. They are typically miles away from each other so I will talk about the branch first.
Head into Milchar and make your way through the tunnels to the Grove of Reflection. When you are there, you will find a pedestal in the centre of the Grove, activate it and a cloud of smoke will come out of it. The cloud turns into a human form, which is actually a double of yourself. You must kill the clone and then you will be able to take a branch from the Tree. Your clone will also have a nice sword if you want to pick it up (Shadowrend). Now, it is time to get back out (to your left press the green button to open the wall) and head to the other location.
In the Howling Halls, I just decided to blast my way through there killing anyone that got in my path. I followed the marker all the way through the halls and into the quarters of Ciirta. To enter these quarters, there is a very hard lock to pick so I saved before trying it just in case I ran out of lockpicks. I did this lock manually rather than trying the auto-attempt due to its difficulty. Inside the room, you will find Ciirta and another Apostle. Kill her and loot the eye from here corpse.
Now you have both of the items, fast travel back to Dyus at Knifepoint Hollow so that he can create the staff for you. With your brand new staff, you need to head back to new Sheoth to use it at the Font.
However, when you arrive, it is not possible to activate the Font. Speak with Haskill and he will tell you to head inside the Fountainhead so that you can cleanse the pools of Madness. Inside the Fountainhead, there are two types of Gnarls, tame and wild. The hostile ones will glow a silvery aura to help you know which to kill. The others will be a typical brown colour. These Gnarls can open the crystal-encased Order doors. When you get to a pool, kill the priests around it and the shards will disappear cleansing the pool. There is a minor pool you come across first and then you need to cleanse the Mania pool and Dementia pool. The markers on your map will direct you to each of these. Again, kill all of the priests around the pool when you get to it to cleanse it. Repeat the process again for the other pool. Syl will be present at one of them if you have followed all my solutions and you will kill her along with the other priests. Now that you have cleansed the pools, you can finally head back into the throne room by following your marker and activate the Font of Madness.
With this done, a Golden Saint, will enter the room reporting Knights in the Palace grounds. Follow the Saint outside to see two Obelisks spawning Knights. Similar to the previous Obelisk you came across, just grab 6 hearts (3 for each Obelisk) from Knights you kill and place them in the Obelisks. When you have deactivated both Obelisks and killed all enemies, Jyggalag will appear in a flash of blue light. Normally, this would be a tough fight but thanks to the lowered difficulty, just head straight up to him and give a good load of slashes with your sword and he will drop dead. His face will appear in the sky near the body and he will give a final speech. The achievement unlocks right here.
After the speech, you can go back to Haskill to be given your new title or you may have already ejected the disc as soon as the achievement unlocked.
I hope that these solutions were easy to understand and if you have any problems, please let me know.