Do you have to use the Irelia legend during finalization of a chain item which targets a unit? How does the chain work in this situation? Can I still use the legend after finalization?
No, you do not have to use Irelia's legend ability during the finalization of the spell or ability that triggers it.
How the Trigger Works
Irelia's legend ability (Blade Dancer) states: "When you choose a friendly unit, you may exhaust me and pay [A] to ready it."
Because this ability says "you may," it is an optional triggered ability. According to the Golden Rules, you are not required to place optional triggered abilities on the chain. You decide whether to trigger it at the moment the condition (choosing a friendly unit) is met.
The Chain and Timing
When you play a card or activate an ability that chooses a friendly unit:
- Finalization: During the finalization step of your action, you choose the friendly unit. This is when the condition for Blade Dancer is met.
- Triggering: If you choose to use Blade Dancer, you place it on the chain as a Pending Chain Item alongside your original action.
- Ordering: Since you control both the original action and the legend trigger, you decide the order in which they are placed on the chain (Rule 376.3.b).
- Resolution: The chain resolves in Last-In, First-Out (LIFO) order (Rule 336.1).
- If you placed Blade Dancer on top, it will resolve first, readying your unit.
- Then, your original action will resolve.
Can you use it after finalization?
No. Once a spell or ability has finished finalizing and has begun to resolve, the opportunity to trigger Blade Dancer from that specific "choose" action has passed.
If you do not choose to trigger Blade Dancer during the finalization step of the action, you cannot "add it back" to the chain later in response to that same action. The trigger condition must be met and the choice made during the finalization of the card or ability that causes the "choose" action.
Disclaimer: This is my interpretation based on the provided rules for triggered abilities and the chain. No official FAQ exists specifically for the timing of "choosing" versus "triggering" in this exact sequence beyond the general guidance on optional triggers.