The Grand Conjuration meaning

Well, reading the lyrics, avoiding the video clip (because it doesn't make so much sense to me, mostly the toilet at the end :erk:), and according to the album's concept, I'd say it's about the Devil taking possession of someone, simply.

First person talking
"Majesty, (...) pour yourself into me" (The Devil possessing the character)
"Wield your power, (...) stare me down to the ground"
"This poetry, our blasphemy" (The Devil and the character are... one)

Second/Third person talking
"His orders in your mouth, a decree for domination" (The Devil talks with the character's mouth)
"Injected seeds of vengeance, usurper's eyes of the powerless" (Injected seeds of vengeance are the devil, the powerless is the character)

"Tell me why love subsides in the light of your wishes" (Some interpretations to do with this... It could be the main character who is possessed by the demon and has the feeling that what the devil wishes him to do is not hateful or bad)