Lucy, I also love Opeth. But ok here's my story...
I 'think' I Therionized my girlfriends sister. I forget which song I was playing, I think it might've been something off of Vovin, mayhap Clavicula Nox. But my gf's sister said "Hey, this is pretty awesome, who is this?" And I was pretty proud of myself, because she normally listens to Disturbed and other bland radio metal. So I thought that was rad.
Now as for me, I was Therionized by one of my punk friends. One day we were hanging out at a buddy's house, and my punk friend and I were hungry, so I went w/ him to a drive through, and he put in a mixed CD, and "To Mega Therion" came up. What immediately got me hooked was the soprano singer. Just hearing her against a backdrop of metal guitars and choir singing got to me. Before then, I was mostly a thrash metal fan among other genres, listening to Slayer, Sacred Reich, Rammstein and the like, but this new genre of classical metal music quite literally sung to me, heh. I also listen to classical music, (Gustav Holst, Wagner, Carl Orff, etc...) so I loved how Therion combined metal and classical. Now Therion is my absolute fave band.