Complex sentence: Varys promises Tyrion that his spies will find the leader of the Sons of the Harpy, when they discover that somebody has set fire to all of the ships in Meereen's harbor.
Split into simpler sentences: Varys promises Tyrion that his spies will find the leader of the Sons of the Harpy. They then discover that somebody has set fire to all of the ships in Meereen's harbor.