Spell Mastery is very easy for most Commander decks to have active, especially token decks that often produce tokens with instants/sorceries
For those who aren't interested in milling their opponents, when used to mill yourself, high-toughness creatures in a Phenax deck can easily enable self-mill win conditions like Laboratory Maniac and Thassa's Oracle.
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It provides another angle of attack and a strong win condition in an archetype that can often spin its wheels to no effect
10 Chandra, Fire Of Kaladesh The Emblem Is The Goal Chandra, Fire of Kaladesh is a low mana value commander that is all about low mana value red spells
If you want to pay 7 life to activate Griselbrand, but your life total is 6, you don't have enough life to pay the cost