Complex sentence: Treniota usurped the throne and reverted the nation back to paganism, but only ruled for a year before being deposed and replaced by Vaišvilkas, the younger son of Mindaugas.
Split into simpler sentences: Treniota usurped the throne and reverted the nation back to paganism. However, he only ruled for a year before being deposed by Vaišvilkas, the younger son of Mindaugas.