Complex sentence: They came into conflict with another faction, The Mountain (Montagnards, a more radical group within the Jacobin Club) which led to the fall of Girondists and their mass execution during the Reign of Terror.
Split into simpler sentences: They came into conflict with another faction, The Mountain (Montagnards, a more radical group within the Jacobin Club). This conflict led to the fall of Girondists and their mass execution during the Reign of Terror.