Complex sentence: It unfortunately spurred a much greater counter-massacre (led by William Fraser), it is believed that as many as 300 Aborigines may have been shot in retaliation, that would end in the extermination of the entire Yeeman tribe and language group by 1858.
Split into simpler sentences: It unfortunately spurred a much greater counter-massacre (led by William Fraser). This retaliation resulted in the extermination of the entire Yeeman tribe and language group by 1858.