Therion is just an amazing band, but the symphonic "death opera" sound is more of an aquired taste in my opinion. The riffs in most of they're songs are solid, catchy, shit that you can't help but headbang to. Even though Therion is now a symphonic metal band, they have thier begginings in the vein of extreme Death Metal. The first few albums they put out were pure death metal, but they were never recognised until they solidified a new sound and adopted a somewhat different style.
Have a listen to these guys, i suggest they're covers of Metallica's "Fight Fire With Fire" and the classic Hammerfall hit "Crazy Nights".
Horns Way Up