When do you make choices for 'may' triggers - when they go on the stack or when they resolve? Specifically for Draven attacking Reaver's Row.

Ruling: 'May' choices and any associated costs are decided and paid entirely when the ability resolves, not when it goes on the stack.

Sequence:

• Attacking player triggers go on the chain first
• Then other players' triggers (in multiplayer)
• Then defending player triggers
• Each player orders their own triggers as they prefer
• When a 'may' ability resolves, the controller decides whether to use it and pays any costs at that time

Nuances:

• For Reaver's Row, you pick the target unit when the ability goes on the chain, but choose whether to move it when the ability resolves
• For Draven attacking Reaver's Row, Draven's ability goes on the chain, but you can wait until resolution to decide whether to pay, meaning you can see if the opponent uses Reaver's Row to retreat first
• If Reaver's Row triggers, it counts as a combat for Draven's trigger purposes even if the unit retreats