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So tyrants of blood says:

After a model from this battalion has fought in the combat phase for the first time, you can pick another model from the same battalion that has not yet fought in that combat phase and is within 3" of any enemy units. That model fights immediately, before the opposing player picks a unit to fight in that combat phase. That model cannot fight again in that combat phase unless an ability or spell allows it to fight more then once.

Leave none alive says:

You can use this ability at the start of the combat phase. If you do so pick 1 friendlyy Reapers of Vengeance daemon unit wholly within 8" of a friendly daemon model with this command ability. After that unit has fought in the combat phase for the first time, if it is within 3" of an enemy unit it can immediately make a pile-in move and then attack with all of the melee weapons it is armed with for a second time.

The way I interpret this is as they both trigger together, you have to choose which one to trigger first and when you trigger one the condition for the other ability is no longer there. (after that model has fought for the first time)
So the max you can get is all your thirsters hit in a row and the last one hits twice. or they all hit twice but not in a row.

Or do they mega combo allowing you to hit with all your thirsters twice in a row?

Your interpretation that the trigger is no longer given (after the first time it fights) after you choose to activate "Leave none alive" seems to be correct.

Either chain them together and use the Command Ability on the last one to activate; or use it on all and not chain them together.

thanks.
So I was right that the condition of an ability is checked right before it resolves and not just put on a stack when it triggers.


Automatically Appended Next Post:
So I have been having this discussion on other place to. Where people massively argument that once they trigger the active player gets to choose the order.
There is nothing in the rules that says you go back to check on the condtion again.

So basically, you can hit with all the thirsters and then go back and hit with all of them again. as all these abilities triggered at the same time, you choose the order of in which you resolve them.

The core rules state that any rules which happen at the same time, in this case ‘Immediately’ can be put in whatever order the player activating them wishes.
There’s nothing that can stop you from doing this until they FAQ it.