Complex sentence: "In My Darkest Hour" was written by Dave Mustaine shortly after Cliff Burton's death, who was the bassist for Metallica, which Mustaine was a part of from 1981 until 1983 just before Metallica recorded its debut album "Kill' Em All".
Split into simpler sentences: In My Darkest Hour was written by Dave Mustaine shortly after Cliff Burton's death. Burton was the bassist for Metallica, which Mustaine was a part of from 1981 until 1983 just before Metallica recorded its debut album "Kill' Em All".