Baleful Mastery — ruling 1

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.

Baleful Mastery — ruling 2

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.

Baleful Mastery — ruling 3

If an effect increases or decreases the cost of spells you cast, that cost increase or decrease is applied to the alternative cost you chose to pay. In that case, the cost was still paid for the purposes of the effect, even if you paid more or less for it when it was cast.

Baleful Mastery — ruling 4

If you copy a "Mastery" spell and the alternative cost was paid, the copy will resolve as though the cost was paid.

Baleful Mastery — ruling 5

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

Baleful Mastery : règles, fonctionnement et interactions (MTG)

Baleful Mastery is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

You may pay rather than pay this spell's mana cost. If the cost was paid, an opponent draws a card. Exile target creature or planeswalker.

How it works : Baleful Mastery

Original English text — Wizards of the Coast / Scryfall

#1April 16, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 16, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 16, 2021· Wizards of the Coast
If an effect increases or decreases the cost of spells you cast, that cost increase or decrease is applied to the alternative cost you chose to pay. In that case, the cost was still paid for the purposes of the effect, even if you paid more or less for it when it was cast.
No other card is named in this ruling.
#4April 16, 2021· Wizards of the Coast
If you copy a "Mastery" spell and the alternative cost was paid, the copy will resolve as though the cost was paid.
No other card is named in this ruling.
#5April 16, 2021· Wizards of the Coast
In a multiplayer game, you choose which opponent takes the prescribed action as the spell resolves.
No other card is named in this ruling.