Complex sentence: Although Thomas was named Lord High Admiral, as a concession, he was consumed by jealousy of his brother's power and influence and worked to unseat and replace his brother as Lord Protector.
Split into simpler sentences: Although Thomas was named Lord High Admiral, as a concession, he was consumed by jealousy of his brother's power and influence. Thomas's fervent desire was to unseat and replace his brother as Lord Protector.