Originally Posted by
Cleretic
I still don't think what they were sacrificing needs to be specific or is necessarily important. With this, I always step back and break it down on what the story needs that third sacrifice to be to make sense.
1. It can't be an option that was always safe and available, or else the Convocation would've looked stupid for not doing it the first time.
2. It can't be an option that's just unobjectionably safe and preferable, or else Venat's group would've looked stupid for objecting to it.
3. It still has to be goddamn huge, or else the plan doesn't make sense on both a narrative and logical level.
We can rule out that it involved sacrificing more Ancients, because that was never how it was described by either Fake Hythlodaeus or Venat's group. But beyond that, all we have is its weight; that it's a sacrifice big enough to sound that final alarm, and force that argument of 'save our past versus save our future'. I don't think we'll ever get an answer to what that is, nor do I think the story would actually benefit from them being specific, because ultimately, this comes down to an uncertain moral question that's intended to not have a 'right answer'. Giving us objective specifics would just lead to people deciding there is a more objective and specific answer.
The third sacrifice was whatever would have made you in that context either go 'this is too far and needs to be stopped by any means' or 'I can understand why people would object but it's for the greater good'. I have my own internal answer to that, but I won't share it, because if I do I know that's just going to lead to people trying to tear down my internal reading that works for me, rather than trying to find an answer that works for them.