Complex sentence: He has been working as drum tech for Therion, after he left the band, but he rejoined Therion in 2017 November, replacing Johan Koleberg who left the band almost precisely one year before.
Split into simpler sentences: He has been working as drum tech for Therion, after he left the band. He rejoined Therion in 2017 November, replacing Johan Koleberg who left the band almost precisely one year before.