Dragonfires

"But should these Dragonfires fail, and should no heir of our joined blood wear the amulet of Kings, then shall the Empire descend into darkness.... "

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 of the Septim bloodline] at the beginning of his reign. This ensures that the daedra from the realms of Oblivion will not be able to travel to Cyrodill. If an Emperor dies and has no heir,[as happens at the beginning of TES4:Oblivion] then the Dragonfires will go dark, and the Empire will become vulnerable to an attack by the daedra.