This song, heard in Season Five promos, is actually an old spiritual. CW senior VP and creative executive Chris Donovan, decided that using the old spiritual "O Death" (heard in "Brother Where Art Thou") fit well with the themes of Season 5. He contacted Jennifer Titus, a well-known name around the CW, about singing the song. She recorded a preview of the song on her iPhone and sent it to him.
Donovan was extremely impressed, and upon a slight re-write of the lyrics to fit the show better, Titus came into the studio and recorded the official version. CW Staff members recorded the humming heard in the song, and Leonard Richardson, the executive in charge of music on the CW, mixed the track.Source.
Won't you spare me over another year
Well what is this that I can't see
With ice cold hands takin' hold of me
When God is gone and the Devil takes hold
who will have mercy on your soul
No wealth, no ruin, no silver, no gold
Nothing satisfies me but your soul
Well I am Death, none can excel
I'll open the door to heaven or hell
My name is Death and the end is here.