Complex sentence: In 1980, he left Black Rose and joined the punk-metal band Brats where he met the three musicians (Hank Shermann, Michael Denner, Timi Hansen) that would join with King Diamond in 1980 to become Mercyful Fate.
Split into simpler sentences: In 1980, he left Black Rose and joined the punk-metal band Brats where he met Hank Shermann. This band included Timi Hansen and the mentioned musicians would join with King Diamond in 1980 to become Mercyful Fate.