In the last book Severian goes to the Otherworld (Yesod) which is a planet-ship in deep space, to get there he travels across space and time inside a living spaceship Tzadkiel that is several times compared to a tree. The whole story is essentially a time travel paradox, where people/gods/angels from the future are rewriting events of the past in order to get Severian to bring the New Sun. He brings the New Sun, he brings the Dawn. Being an executioner he is uniquely qualified to carry out the death sentence of the Old Urth.

Severian is the chosen one, he is the Christ figure and his ultimate fate is to destroy the world and remake it anew. The story is the autobiography of Severian who is orphaned bastard who is raised by a disreputable guild of torturers that wear black, and his rise to the Phoenix Throne. The story is set in the far future of our Earth/Urth, and due to humanity becoming decadent and the crimes of the tyrant monarch Typhon, alien gods (who are actually genetically engineered humans from a previous universe) have placed a black hole/'black pit'/'black worm' inside the sun, which absorbs much of the sun's output causing Urth to cool significantly, if nothing is changed, Urth will soon become an ice planet. Wolfe uses the same idea that Tolkien did, that he somehow came to possess a manuscript from another time and translated it into English. I think there are a lot of plot parallels between The book of the New Sun and ASoIaF. For years and years to come, I think, we'll see inferior imitations of Wolfe's masterpiece decked out with the phrase 'In the tradition of Wolfe', for that is the tribute all great originals are inevitably paid' 'One of the great science fantasy epics of all time.'

Wolfe's the book of the new sun