I'm fairly sure that The Grand Conjuration is one of Mikael's favorite songs by the band, at least that's what I've collected from facts.
They have played it quite a lot live (compared to some other songs from GR), and I remember seeing an interview with Mikael where he talked about how evil the song was, and that he was very happy about writing it. (or something like that)
As for my own thoughts on the song: It's really great. Might get a bit repetitive, but I don't really listen to it on repeat, so it's not really an issue.