So as the title.. do you play this:

1: in a round and it hangs around for a few rounds until an ally leaves play and then you get your response.

Or

2: as your ally dies, you instantly play the card (surely no action window) then get your response

Or.. as i suspect:

3: you need to predict when you will be losing an ally, play the card in the action window at the start of the phase, have your ally die and get your response before the end of the phase (if ally survives, Card is wasted)

Thoughts please..

Thank you.

It's a Response, so it's #2. Responses are only eligible to be played the moment that their trigger occurs. For Valiant Sacrifice, this is immediately after an ally leaves play. You don't need an action window-- actions windows are for Actions! Responses don't need them, they only need their trigger.

Thanks I had thought you needed to play the card as an event in an action window, then the response occurs upon card text-happening, so thats good to know thank you !!