Board Thread:Lore Discussion/@comment-68.45.229.57-20130720115140/@comment-6881629-20130720115946

The full prophecy Harkon was going for permanently blocks out the sun and involves Serana's (or her mothers) death, copy/paste from the wiki page -

"The inscription denotes a ritual that requires a Daughter of Coldharbour as a sacrifice and the bow of Auri-El to permanently purge the world of sunlight."

We're not fulfilling the prophecy as Harkon would have just a smaller ritual based on it to get the effect for a limited time.