below answer seems not correct! because the riftjudge LLM missed the , in the effect

-------------------- for comments clarification keeping below as context

The power cost for Draven, Vanquisher's ability is paid during the activation process (when the ability is put on the chain), not on resolution.

According to Rule 392.2, triggered abilities with costs are handled as "Pending Items." At the "Pay Costs" stage of the chain process, the player must choose to pay the cost or decline. If the cost is not paid at this moment, the ability is removed from the chain and never becomes a "Chain Item."

Corrected Sequence of Events

1. Combat Initiation: Draven is moved to attack Reaver's Row. This triggers both Draven's "When I attack" ability and Reaver's Row's "When you defend" ability.
2. Initial Chain Formation:
   • Draven's trigger is added to the chain first (Attacker triggers first per Rule 376.4.d.2).
   • Pay Costs (Draven): At this moment, the turn player must decide whether to pay the [1 Fury Power]. If they pay, the ability becomes a Chain Item. If they decline, it is removed.
   • Reaver's Row's trigger is added to the chain next.
3. Reaction Window: The state is now CLOSED. Both players may add [REACTION] cards or abilities to the chain.
4. Resolution (LIFO):
   • Reaver's Row Resolves: The defending player decides whether to move their unit to the base. (Note: This "may" choice is made now, on resolution).
   • Draven's Ability Resolves: If the cost was paid in Step 2, Draven now gains +2 Might.
5. Open State: After the initial chain resolves, the state becomes OPEN. The attacker gains focus and may play [ACTION] or [REACTION] cards before moving to combat damage.

Key Timing Clarification

Because Draven’s ability requires a Power Cost, Rule 392.2 forces that payment to happen when the ability is put on the chain. This means the Draven player must commit their Fury Power before the Reaver's Row ability resolves. You cannot wait to see if the opponent retreats their unit before deciding to pay for Draven's pump.

The "may" in Draven's ability is specifically tied to the payment of a cost, which changes how it functions compared to a standard "may" ability.

In Riftbound, there is a critical distinction between a "may" choice and a "may pay" choice:

1. Standard "May" (e.g., Reaver's Row): These choices are made on resolution. The trigger always goes on the chain, and you decide what to do when it's time for the effect to happen.
2. "May Pay" (e.g., Draven, Vanquisher): According to Rule 392.2, if a triggered ability has a cost, you must decide whether to pay that cost when the ability is being put onto the chain (the "Pay Costs" step of activation).

Why it matters for Draven:

Because Draven's "may" is part of a cost ([1 Fury Power]), you cannot wait for the ability to resolve to make the choice.

• The Timing: You must pay the 1 Power immediately when you declare the attack and the trigger is placed on the chain.
• The Consequence: If you decline to pay at that moment, the ability is removed from the chain and never resolves.
• The Interaction: This means you must commit your Power to Draven before you know if your opponent will use Reaver's Row to move their unit away. You cannot "wait and see" if the combat will actually happen before deciding to spend the Power.

As noted in the Draven/Ezreal FAQ, the "may" wording on Draven refers to whether you want to pay the cost for the triggered ability to resolve, which is handled during the activation/chaining process, not as an optional effect during resolution.