Complex sentence: In August 1988, Megadeth appeared at the renowned Monsters of Rock festival at Castle Donington in the United Kingdom performing to an audience of 107,000 people, with a show that also featured Metallica drummer (and former band mate) Lars Ulrich making a guest appearance.
Split into simpler sentences: In August 1988, Megadeth appeared at the renowned Monsters of Rock festival at Castle Donington in the United Kingdom performing to an audience of 107,000 people. One of these shows also featured Metallica drummer (and former band mate) Lars Ulrich, who made a guest appearance.