This is story related and cannot be missed. However, that doesn't mean that it's obvious how you attain it!
It is also the achievement for completing the game so if you haven't got there yet and don't want SPOILERS then stop reading now!
Once you have become the Seneschal you will be left in the clouds with the throne. All you can do at this stage is sit in the throne whereupon you are returned to Gransys as an invisible figure. While it's fun to grab crates/barrels/people and carry them around there's not a whole lot to do except play the poltergeist. So exit out of Gransys through the nearest door.
You will then be returned to the throne cloud. Take out your Godsbane (in tools) and hit X. You will be rewarded with the true ending and the ability to start NG+. Enjoy :)
tldr... Stab self with Godsbane in the cloud throne room