So for starters - Lets say I have an epic in play like History of Benalia and I also have an Arenson's Aura - When is my last oportunity to sacrifice the History of Benalia with Arenson's Aura? Can I sacrifice it as a response to it's stage III sacrifice trigger going on stack?
History of Benalia will be sacrificed as a state-based action if it has three or more lore counters on it and a chapter ability of that Saga (not: epic) isn't on the stack or waiting to go on the stack (C.R. 714.4). Until then, you will still get priority to activate abilities, including Arenson's Aura's first ability, before History of Benalia's III ability resolves (C.R. 117.1b).
Can I sacrifice it as a response to it's stage III sacrifice trigger going on stack?
Yes.
FYI
Saga cards don't actually have a 'sacrifice trigger', although it certainly feels like it.
The sacrifice is actually done as a State-based Action, right after their last chapter resolves (or gets countered).
https://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/817922-sacrifice-triggers#c1
But I've got a few more questions
So for starters - Lets say I have an epic in play like History of Benalia and I also have an Arenson's Aura - When is my last oportunity to sacrifice the History of Benalia with Arenson's Aura? Can I sacrifice it as a response to it's stage III sacrifice trigger going on stack?
FYI
Saga cards don't actually have a 'sacrifice trigger', although it certainly feels like it.
The sacrifice is actually done as a State-based Action, right after their last chapter resolves (or gets countered).
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules