As for origin, I thought from the 1st run that it's something they notice and settle around it: a big temple-like building next to the water source, of course they would settle around it. The temple make for a dandy place to store long-term storage stuff. Why would anyone think the tribe built it?
That's just swapping one bizarre, illogical conclusion with another bizarre, illogical conclusion. If the tribe didn't build it, who did? Who decided to build a pyramid in the middle of bumblefuck nowhere and do absolutely nothing with it but stuff it full of Indiana Jones-tier traps and pitfalls? This isn't Cairo, it's fucking Southern California.
So the tribe comes across it, and instead of doing what most would do when faced with a building that's nothing but a giant death trap and completely unknown to them (i.e. stay the fuck away from it), they decide to turn it into an elaborate testing ground to see who the chosen one is. This would require them to send teams of people in to scope out the entire thing, ensure that it's at the very least fair to whoever's having to go through it (likely losing men to any trap they fail to detect, as well as to the various critters inhabiting the place), and do so in a way which doesn't do irreparable damage that'd ruin the whole thing.
All of this instead of, say, just training someone up sufficiently via conventional methods to ensure they know what they're doing.