When attacking with Draven, Vanquisher, when must the power cost for his triggered ability be paid - on activation (when the ability goes on the chain) or on resolution?

Ruling: Draven's power cost is paid ON RESOLUTION, not when the ability goes on the chain. The "may pay" cost appears in the ability's effect text (after the comma), making it a "cost within instructions" that is decided when the ability resolves.

Sequence:

• Draven attacks, triggering his "When I attack" ability
• The triggered ability automatically goes on the chain (no cost paid yet)
• Defender's "When you defend" triggers are added to the chain
• Reaction window opens for both players
• Chain resolves in LIFO order (last in, first out)
• When Draven's ability resolves, the controller decides whether to pay the 1 Fury Power
• If paid, Draven gains +2 Might for the turn

Nuances:

• This differs from Deflect costs, which must be paid when the ability goes on the chain
• Because the cost is paid on resolution, you can wait to see if the opponent uses defensive abilities (like Reaver's Row moving a unit away) before deciding whether to spend the power
• The "may" in Draven's ability is part of the effect instructions, not a separate cost-payment trigger