The Dragonfires in the Temple of the One go out upon the death of each Emperor, and must be lit anew by each new Emperor (who must be a Dragonborn) upon their coronation. Akatosh created the Dragonfires with a burning handful of his heart's blood. The Dragonfires themselves are the ward against the planes of Oblivion; though it has been suggested several times in the Main Quest that it protects against all immortal realms. While they are lit, Daedra cannot exist permanently in Tamriel and portals to Oblivion are not possible.
In the beginning of the story, the Dragonfires go out at the Emperor's death and are relit for a few seconds at the end of the Main Quest (where Martin destroys the Amulet of Kings to relight them and thus turns into the Avatar of Akatosh). After the Main Quest has run its course, the Dragonfires have been replaced by a statue of the Avatar, possibly symbolizing the supported idea that Oblivion could never again reopen in the mortal world.
The Amulet Of Kings and the Dragonfires seem to be parts of a whole - the Amulet of Kings was formed from the "tangled skeins of Oblivion" and the sinews of Akatosh's own heart. The Amulet of Kings must be available to light the Dragonfires, and it appears that the Dragonfires cannot burn if the Amulet is destroyed.
Until the records so far, the unlit Dragonfires have been the center of two major plots that eventually led to worldwide crisis, almost cabaple of shaking the very fabric of reality beyond repair. The oldest, in 2E 582, followed the Interregnum, a lapse of time with no Emperor on the Ruby Throne, when the Colovian Duke of Chorrol, Varen Aquilarios, assumed the role of Emperor only to be deceived by Mannimarco, who used the Dragonfires and the Amulet of Kings to shatter the last barriers between Mundus and Oblivion, allowing Molag Bal's plan to drag the planet into Coldharbour. The latest, in 3E 433, when the assassination of Emperor Uriel Septim VII eventually led the Dragonfires to be unlit, but differently from the Planemeld, Mehrunes Dagon actually tried to step physically into Tamriel, only being stopped by the ultimate sacrifice of Martin Septim and the birth of a new covenant.
- "So long as you and your heirs wear the Amulet of Kings, than shall this Dragonfire burn -an eternal flame- as a symbol to all men and gods of our faithfulness..." - Trials of St. Alessia, Imperial City loading screens.