Lyric discussion by Stein 

Cover art for Thy Horror Cosmic lyrics by Black Dahlia Murder, The

No, this song doesn't have anything to do with Satan. It is about the Great Old One Cthulhu.

Elements that confirm this:

  1. The Lord below is sleeping, and the cultists wish he could rise from his eternal sleep - Satan, in christian mythology, is not sleeping, he's active

  2. He has beeing sleeping for vast aeons - a word frequently used by H.P. Lovecraft in his short stories

  3. He stains this planet with visions of murder and hate - Cthulhu, in the short stories, drive many people all over the world insane through dreams

  4. Sea soaked perversion - confirms he inhabits the ocean, unlike Satan who inhabits Hell

  5. We pray for the earth to open, free you of your ageless tomb - this confirms the Lord Below inhabits the Earth physically, unlike Satan who, according to christian mythology, is in Hell, which is not a physical place on Earth. Cthulhu lies trapped in his pacific sunken city of R'lyeh

  6. And last - Oh, Great Old One, I scream your return - Cthulhu is a Great Old One, perhaps the most powerful of them, he's a priest of Azathoth.

this song is epic. theres a fuckin billy and mandy episode about cthulhu haha funny shit.