Complex sentence: Deputy to the Great Sejm, and most infamously, the Grodno Sejm: for his actions during the latter he is remembered as one of the most prominent collaborators with the foreign partitioners of Poland.
Split into simpler sentences: Deputy to the Great Sejm, and most infamously, the Grodno Sejm. His actions there earned him the memory of one of the most prominent collaborators with the foreign partitioners of Poland.