So just curious we are playing this right. If they are both deployed at a planet, one of the daemons attack, Zarathurs would increase the damage to 3. On the next action window, the daemon could be sacrificed for 3 more damage? Thanks.

Yes Zarathurs interrupt could be used for both triggers.

*Zarathur's interrupt.

Is it legal to first attack with Flamers, deal damage, and just after use their skill in action window to sacrifice and deal the same ammount of damage again?

Is it legal to first attack with Flamers, deal damage, and just after use their skill in action window to sacrifice and deal the same ammount of damage again?

After 3.2.6 but before 3.2.7 There also is an Action Window where his ability can be used. So yes.

However the ammount of damage dealth is not perse the same.

Do note that the player with initiative has to declare the action first. Or so I believe this is the case.