Because I Have Willed It is not legal in any format.
Rulings
2017-06-13
Abilities that reduce the cost of a spell apply after abilities that raise them. For example, Borrowed Malevolence with both modes chosen costs {1}{B} to cast if this scheme is face up.
2017-06-13
Effects that reduce the generic mana cost of a spell can't reduce that spell's colored mana requirements.
2017-06-13
If an instant or sorcery spell has {X} in its mana cost, choose the value for X first, then reduce the cost by {1}. For example, Volcanic Geyser with X chosen as 4 costs {3}{R}{R} to cast if this scheme is face up.
2017-06-13
For this scheme's second ability to trigger, the players on the opposing team must have cast four spells total among them.
2017-06-13
If an opponent cast a spell during the opposing team's turn and then left the game, that spell counts when determining whether the last ability of this scheme triggers.
2017-06-13
The second ability only checks that four or more spells were cast. It doesn't matter if those spells are countered or fail to resolve for other reasons.
2017-06-13
In a Supervillain Rumble game, the last ability of this scheme triggers at the beginning of each opponent's end step if that player alone has cast four or more spells that turn.