Complex sentence: Approximately 300 people were in the club when the blaze began when a performance artist was tossing cold-flame pyrotechnics into the air, the firework hit the plastic covering of the ceiling, igniting.
Split into simpler sentences: Approximately 300 people were in the club when the blaze began when a performance artist was tossing cold-flame pyrotechnics into the air. The fireworks hit the plastic covering of the ceiling, igniting.