Who sings 'Embrace the Heresy'?
Solution 1:
The vocalist goes by the name Saiyaka and she is a graphic designer for Riot Games. She posted here on her tumblr about it, along with a link to the following video:
You can find other posts on the official boards referring to her singing here and here:
Fun fact: The graphic designer who sang this song also made the unique targeting reticle for Diana's Q. :)
In addition, you can check the official League of Legends soundcloud here for the song.