Complex sentence: Later in 1996 Nicola Trier, aka Morrigan follows the duo as a bass player and completed the trio that created the first two and important albums of the band that called their own style "Dark Metal".
Split into simpler sentences: Later in 1996 Nicola Trier, aka Morrigan, joined the duo as a bass player, forming the core trio that created the important first two albums of the band. For the first two albums, the band called their own style "Dark Metal".