Complex sentence: The Canadian Party continued to exist as a loose alliance after the election, and Lt. Governor Archibald warned Macdonald at one stage that they were plotting the "extermination" of the Metis.
Split into simpler sentences: The Canadian Party continued to exist as a loose alliance after the election. At one stage, Lt. Governor Archibald warned Macdonald that its members were plotting the "extermination" of the Metis.