Ahh. Not too much, hence why it hasn't been publicly released yet.
It gives them a temple-like building at Mysticism (with the requisite +1 happiness from the Religion civic) and makes their sacrifical altar give them +1 happiness with their Blood and Sacrifice civic and +2exp to melee units.
Lastly, when the Cualli have researched Way of the Wicked, all of their units get a 25% chance to capture a slave in combat under Blood and Sacrifice, allowing them to keep using their custom civic instead of swapping to slavery without overpowering them by delivering the ability at the start of the game. The promotion disappears if they swap out of B+S and reappears if they swap back, making it just like the Slavery chance's end result.
Basically it's designed to help them close the gap of being agnostic and compared to the Mazatl, who have a bunch of temple-replacements and a far better unique civic.
Unfortunantly, because I have to give civic bonuses by buildingclass and a lot of the civs share the "unique_building_civ1" or whatever the Sacrifical Altar is, you'll see a whole lot of things saying they give +1 happy under Blood and Sacrifice, things like Khadi gates and anything using that building class.