Devastating Mastery

The mana value of a spell on the stack is determined by its mana cost, not any alternative costs you used to pay for it.

If you choose to pay one alternative cost, you can’t pay any other alternative costs. For example, if an effect lets you cast a “Mastery” spell “without paying its mana cost,” you can’t also choose to pay its given alternative cost.

In a multiplayer game, you choose which opponent takes the prescribed action as the spell resolves.