Complex sentence: One vocalist that had the group's attention was Michael Bormann, whom was singing for Letter X and had his own group on the side called Jaded Heart, plus he was previously in High Voltage and the J.R. Blackmore Group.
Split into simpler sentences: One vocalist that had the group's attention was Michael Bormann, who was singing for Letter X and had his own side-group called Jaded Heart. Moreover, he was previously in High Voltage and the J.R. Blackmore Group.