Complex sentence: Richard was arrested and imprisoned in December 1192 by Duke Leopold, who suspected him of murdering his cousin Conrad of Montferrat, and had been offended by Richard casting down his standard from the walls of Acre.
Split into simpler sentences: Richard was arrested and imprisoned in December 1192 by Duke Leopold, who suspected him of murdering his cousin Conrad of Montferrat. Leopold had also been offended by Richard casting down his standard from the walls of Acre.