Deathspell Omega are what fans call orthodox black metal. They take Satan into a theistic context and make music out of it. It might sound weird, but, in all honesty, it isn't --if you think about how many metal bands chose Satan as their main lyrical theme, it makes perfect sense. Are they really serious? I couldn't tell you, but I couldn't care less because I don't listen to Deathspell Omega for their lyrics.
While I've only heard one of the band's records (Si Monumentum Requires, Circumspice), I can attest that you'll be in for a treat if you're looking for something that gets better with each listen. The album isn't particularly accessible for the normal listener, but if your desires match that of a twisted and demented being, you'll feel right at home. I've scared a person or two with the artwork and the song titles. It's all a bunch of nonsense and fun in my eyes. The band doesn't actually take it seriously. ....or do they?