When the unit would be destroyed, as this is when the criteria for the Altar of Khaine is fullfilled.
Oshova
2 2010-07-23 17:52:48
Re: [Rules] Altar of Khaine (6 replies, posted in Warhammer Invasion)
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. 
Oshova