Card

Blasphemous Act

Mana cost {8}{R}

This spell costs {1} less to cast for each creature on the battlefield. Blasphemous Act deals 13 damage to each creature.

Part of 12 known combos, 5 of them two-card. 4 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Repercussion, Toralf, God of Fury // Toralf's Hammer, Fall of Cair Andros, Expedited Inheritance, Soulfire Grand Master
B2 Core — Not in the same deck as its two-card combo partners: Repercussion, Toralf, God of Fury // Toralf's Hammer, Fall of Cair Andros, Expedited Inheritance, Soulfire Grand Master
B3 Upgraded — allowed: Its two-card combos are not early, which Bracket 3 allows
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Repercussion, Toralf, God of Fury // Toralf's Hammer, Fall of Cair Andros, Expedited Inheritance, Soulfire Grand Master (none of them early)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

12 known combos, 5 of them two-card.

2020-11-10 · Wizards of the Coast
The total cost to cast Blasphemous Act is locked in before you pay that cost. For example, if there are three creatures on the battlefield, including one you can sacrifice to add {C}, the total cost of Blasphemous Act is {5}{R}. Then you can sacrifice the creature when you activate mana abilities just before paying the cost.
2020-11-10 · Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Blasphemous Act). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
2020-11-10 · Wizards of the Coast
Although players may respond to Blasphemous Act once it's been cast, once it's announced, they can't respond before the cost is calculated and paid.
2020-11-10 · Wizards of the Coast
Blasphemous Act's ability can't reduce the total cost to cast the spell below {R}.

Rulings: Scryfall · Wizards of the Coast