Maybe us old timers need to scour side quests and flavor text.

[Jandor]

Where do you get the "human sacrifice" bit from? They certainly love to offer people's souls to Ifrit and increase the number of worshippers, but no mention I can recall of physical sacrifice, and there are a number of living tempered people around Zanr'ak – there's a whole encampment of tempered Flames who don't seem to be prisoners.

IIRC the original three primals extreme fights have a human sacrifice component to them.