A brief summary of Therion post-BS:
1993 - Symphony Masses: rather awkward death metal with vague symphonic touches
1995 - Lepaca Kliffoth: rather awkward heavy/death metal with slightly more refined (though still minimal) symphonic touches
1996 - Theli: Their first real symphonic metal album, with opera-style choruses and plenty of keyboard usage (though it gets pretty cheesy at times). One plus to this album over the later ones is that the sound is still raw and crunchy
1998 - Vovin: Their best symph metal album imo, and the first one that's actually a pretty even blend of "symphonic" and "metal" rather than just "metal with symphonic embellishments". There are several nice orchestra-dominated sections, and the melodies are generally very pretty.
2000 - Deggial: Almost identical to Vovin, except the symphonic elements are more mature and atmospheric this time, though the songwriting's kinda inferior.
2001 - Secret of the Runes: Back to a more raw/crunchy sound (though still not as much as Theli and before) while still maintaining the strings/choirs. Pretty solid, though not one of my faves. Great ABBA cover though.
2004 - Sirius B/Lemuria: Probably their most bombastic and production-heavy albums. There's some pretty epic use of orchestras and choirs this time. There's actually a bit of a power metal feel to a lot of it. I think the songwriting on Sirius B is leagues above Lemuria, but stylistically of course they're almost identical.
2007 - Gothic Kabbalah: A transition away from symphonic metal and toward prog metal. If there's any Therion album that sounds like / is influenced by later Amorphis, it's this one (though from what I've heard the bands have never really sounded that similar). First 1/3 or so of this album has really great songwriting, and they actually have some charismatic solo vocals for once, but there's too much material on this album overall imo, and it gets really tedious toward the end.
Hope that helps
