This mission will not be offered until you have reached the Skull Cave in the Hidden City. Once you are there, speak to the mission giver, Q'orianka. She is a shaven-headed woman in a Blue Heron outfit speaking with an archer at the north end of the cave.
This side mission does have a substantial amount of combat, so get your weapons, outfits, potions, special ammo, and skills set up before you leave the cave. The enemies include both native cultists and modern Trinity.
Ready? Go into the lake on the north end of the Hidden City. Swim over to the waterfall. Get out on a ledge to the right of it and axe-climb up to another ledge. Squeeze through the opening to begin.
The first part of the mission is to complete the Path of Huracan challenge tomb. This will unlock:
After that, you'll have some platforming to do to reach the first combat area. It has ten cultists. You begin in stealth and should try to remain in it as long as you can. Booby-trap lures and poison lures are useful.
The second combat area also has cultists. You don't have to fight these guys! Just walk out a bit onto the beam, wait for a couple of enemies to pass by from right to left, then sneak past them. Easy peasy!
The third combat area has about a dozen cultists as you progress down a path. Take it slow, only engaging 2 or 3 at a time, and you should be fine.
The fourth and final combat area has mostly Trinity, with a few cultists. You mostly have to hold your ground while they come at you and fire on you and lob grendaes at you from their position. Take advantage of the cover, break their line of sight, use the molotovs you're provided, and shoot the explosives. Fear arrows are good to use against Trinity. A second wave will drop in when you're close to winning.
When you leave this area, you'll transport back to the Skull Cave. Talk to Q'orianka to complete the side mission, unlock the achievement, and get your rewards. You will then be able to unlock:
Please let me know in the comments if I left out something important. That would do more good for the community than an uncommented downvote would.