Originally formed in 1989 as Burial, like many extreme bands of the late 80s, they played a more death metal-oriented style than black metal. As Burial, they managed to put out a couple of now very hard-to-find demos that showcased their old school sound. But, like many extreme metal bands of this era who were in their early years of existence, Burial experienced several lineup changes that also led to changes in their sound. This, in turn, led to them changing their name to Argentum in 1991.
Argentum's initial sound still contained elements of death metal, but there was a very mystical, occultic atmosphere to their music. Combining doom metal with this old-school black/death, Argentum had a sound that not too many other bands had at the time (perhaps Mystifier is a fair point of reference?). Their first demo (Matter Misericordiae), above, is a solid example of this sound.
Misericordiae and a live demo later that year (1993) led to Argentum's name becoming more well-known in the underground. Still struggling with finding dedicated musicians, a new demo emerged after additional line-up changes as well as further evolutions in Argentum's sound. On 1994's Exothaernium, Argentum is moving away from the old-school black/death of the Burial years and toward a doom/death style with occultic black metal influences. I would say that the evil mysticism in their sound is reminiscent of some of the slower, more dirge-like elements of classic hellenic black metal.
After their 1994 demo and a noteworthy EP, Argentum released their true masterpiece: 1996's Ad Interitum Funebrarum. On this album, Argentum's style reached its peak. A doomy, mystical affair that has been compared to Mystifier or even Tiamat, we've got long, gloomy passages with creative keyboard usage. With lots of Latin and eerie vocals, the overall effect is one that calls to mind dark Satanic rituals performed in abandoned temples.
After that album, things in the band kind of ground to a halt. They went on hiatus in 1997 before breaking up in 2001. They have, however, re-formed in the past few years and are as prolific as ever.