Complex sentence: Sigismund used the killing of his envoy Lancki by the "landmeister's" son as an excuse to invade the southern portion of Livonia with an army of around 80,000, forcing the discordant parties to reconcile at his camp in Pozvol in September 1557.
Split into simpler sentences: Sigismund used the killing of his envoy Lancki by the "landmeister's" son as an excuse to invade the southern portion of Livonia with an army of around 80,000. He forced the competing parties in Livonia to reconcile at his camp in Pozvol in September 1557.