Complex sentence: Fawkes was in charge of the gunpowder in a cellar beneath Parliament, but he was arrested a few hours before the planned explosion, during a search of the cellars underneath Parliament in the early hours of 5 November prompted by the receipt of an anonymous warning letter.
Split into simpler sentences: Fawkes was in charge of the gunpowder in a cellar beneath Parliament. He was arrested a few hours before the planned explosion, during a search of the cellars underneath Parliament in the early hours of 5 November prompted by the receipt of an anonymous warning letter.