If I have 2 units targeted by [whi]Tzeentch's Firestorm[/whi], each with 2 HP, and I have [whi]Altar of Khaine[/whi] in play, when can I activate it?

There is no action window to respond to non-combat damage until after damage has been applied, and once a unit has zero HP, it immediately is destroyed, so if I let the Firestorm resolve, by the time I can play an Action, the unit is already in the discard pile. Do I have to activate the Altar in response to the Firestorm being played, because my unit "would be destroyed" by the spell?

Are the action windows defined in the rulebook the only time that game-triggered actions can be played? Or do those windows refer to only player-triggered Actions and Tactics?

The wording of The Alter Of Khaine says "Whenever a unit would be destroyed . . ."

So your unit would be destroyed by Tzeentch's Firestorm, meaning that you may save it using the Altar. =D

Oshova

Do I activate the Altar in response to Tzeentch's Firestorm though, before my unit has even received damage? Or in the action window after the unit has been destroyed?

When the unit would be destroyed, as this is when the criteria for the Altar of Khaine is fullfilled.

Oshova

But there is no action window for non-combat damage at the time when the unit is being destroyed. By the time I have an action window, my unit has already been destroyed.

1. Firestorm is played
2. Both players can take actions.
3. Firestorm resolves, damage is assigned, damage is applied, all units with < 1 HP are destroyed.
4. Both players can take actions.

Found this from the FAQ post:

Altar of Khaine
Q. When do I need to trigger this Action in order to save my units which are being destroyed?
A. This card has received errata, and is no long an Action. The effect now occurs "when the unit would be destroyed." (FAQ v1.1)

As it is no longer an action, the 1 resource payment would occur [u]as damage is applied[/u], before the action window after damage application.

Yep, thanks. The question was from before they'd addressed it in the FAQ, since the card timing didn't quite line up when it was still an Action.