Complex sentence: The most famous was the Cossack Yemelyan Pugachev, who led what came to be known as Pugachev's Rebellion in 1774, which was ultimately crushed by Catherine's forces.
Split into simpler sentences: The most famous was the Cossack Yemelyan Pugachev. Under this guise, he led what came to be known as Pugachev's Rebellion in 1774, ultimately crushed by Catherine's forces.