He shuts down larger spells, hurts ramp, storm, buyback, my own Equinaut deck (which likes to play and replay creatures), and countless other decks that either want to cast something big or net multiple spells
And the way it dances across the panel, disrupting the saga counter progression, which replicates the duplicated first two steps with their Umezawa's Jitte choices, is perfect
If it's a Saga, put up to three lore counters on it
Mortarion, Daemon Primarch seems like he might be an important character in the Warhammer 40,000 lore
Sure, your Consecrated Sphinx is good to resolve, but before my turn, for the price of one blue mana and an expendable creature, let me get my own straight to the field with Mask of the Mimic as well