I've been listening to them for nearly a decade and only own... two albums, I believe. Secret of the Runes (I bought it mostly because it's been my favorite album since 2006 or something) and Sitra Ahra. I used to illegally download the albums, burn them to CD, and then listen to them with headphones in the dark. Focusing on nothing but the music. I'm still hard pressed to find classical music that has affected me the way Therion has.
Nifelheim is probably the best. I have to put this song on repeat, and I think a part of my soul melts each time I listen to it. I think that's a good thing? And part of it, I think, is the nostalgia aspect. I'm sure I could find plenty of classical music that makes my soul melt, and both Beethoven and Mozart have certainly done it. (I once had a mini-breakdown for an hour after listening to Beethoven because I didn't know what to do with the emotions the 2nd movement of Symph. 7 created.)
So yes. Buy more Therion albums, I shall. When I finally come across the money.
Anyway. I'm putting off my paper. I told myself I'd have a rough draft done today (... haha) and I'm just distracting myself with journals.