The Battlefield

Phyrexian Altar

$51.12

The Great Henge

$59.63

AI DraftFeb 26, 2026

Phyrexian Altar + The Great Henge

Overview

When you cast The Great Henge you first put it on the stack and then determine the spell’s total cost (which includes the reduction based on the greatest power among creatures you control) . You may activate mana abilities (such as Phyrexian Altar’s ability to add mana) only after the total cost is determined and before paying costs . Because the cost-reduction for The Great Henge is locked in when the total cost is determined , activating Phyrexian Altar after announcing the spell cannot change the already-locked cost-reduction; you can, however, activate Phyrexian Altar as a mana ability to produce mana to pay the locked cost . The official ruling notes that moving the spell to the stack can change the value used for the reduction and that the new value is used; but that check happens as part of determining the cost when the spell is cast [Ruling 2025-10-02].

Step 1

Announce you’re casting The Great Henge and move it from your hand to the stack .

Step 2

As part of casting it, determine the total cost, including the reduction based on the greatest power among creatures you control at that moment; that total cost is then locked in . The official ruling confirms that any change caused by moving the spell to the stack is taken into account when determining the reduction [Ruling 2025-10-02].

Step 3

If the total cost includes a mana payment, you may now activate mana abilities (such as Phyrexian Altar’s ability) to produce mana before paying the locked cost .

Step 4

Activate Phyrexian Altar (sacrificing a creature as its cost) to add mana if you need mana to pay The Great Henge’s already-locked cost. The sacrifice affects the game state after the cost has been locked in, so it does not change the Henge’s cost reduction .

Step 5

Pay the locked total cost and finish casting The Great Henge. When it resolves it will enter the battlefield; any subsequent changes to creatures do not retroactively change the cost you already paid .

Resolution

You cannot use Phyrexian Altar’s sacrifice activation (performed after The Great Henge has been announced and moved to the stack) to retroactively change The Great Henge’s cost reduction. The Henge’s reduction is determined and locked in during cost calculation ; you may then activate Phyrexian Altar as a mana ability to produce mana to pay that already-determined cost . The ruling clarifies that any change caused by moving the spell to the stack is taken into account when determining the reduction [Ruling 2025-10-02].

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