Complex sentence: Psycroptic was formed in 1999 by brothers Dave (drums) and Joe (guitar) Haley who had previously been involved in a band called Disseminate, which had recorded a demo in 1998 before disbanding.
Split into simpler sentences: Psycroptic was formed in 1999 by brothers Dave (drums) and Joe (guitar) Haley who had previously been involved in a band called Disseminate. That group had recorded a demo in 1998 before disbanding.