Complex sentence: It is supposed that this demo was actually released in 1999; the demo "Herbstleyd", which Kanwulf claims to be released in 1993, was probably not released before this date.
Split into simpler sentences: It is supposed that this demo was actually released in 1999. The demo "Herbstleyd", which Kanwulf claims to be released in 1993, was probably not released before this date.