Torygg feared the Dominion as much as he desired emancipation. He knew the Rift would receive Skyrim like a tomb, if he waged open rebellion on the Empire; and there was no time for diplomacy. He approached the Emperor's stratgist, General Tullius, for a ruse. Torygg agreed with the plot to cosign his life to tragic, national icon, as remission of Skyrim's treason.
Tullius crafted the High King's assassination out of the Arcturian Heresy. (Know the Heresy is the Ritual his cult simply understand. It is not a book, collection of knowledge, or theology. If anything, the Heresy is anthropology.) There was one clue: King Olaf's burning verse resurfaced, via unknown agency, the eve of his effigy's ritual slaying. Tullius hid this theme for us to follow.