I’m sure that the thinking here is “the questline is in my hand at the start of the game, not my deck, so Benedictus will still activate because all spells in my deck are Shadow,” but if this works like Genn and Baku, it checks your decklist, not your deck, so this most likely won’t work. (You cannot run quests in an Even deck, nor can you run C’thun the Shattered in an Odd deck. You can, however, run C’thun the Shattered in an Even deck)
I’m sure that the thinking here is “the questline is in my hand at the start of the game, not my deck, so Benedictus will still activate because all spells in my deck are Shadow,” but if this works like Genn and Baku, it checks your decklist, not your deck, so this most likely won’t work. (You cannot run quests in an Even deck, nor can you run C’thun the Shattered in an Odd deck. You can, however, run C’thun the Shattered in an Even deck)