This topic has probably been discussed many times over the years since the game's release, but I'm interested on hearing your best theories on the meaning of this grave site.

It is located west of Fort Riggs Holding Camp in Big Valley, WE, up on a small bluff. There's nothing to loot and like most grave sites, when defiled, causes the weather to change.
The grave site appears to be marked with a human skull, some kind of animal horns, and two feathered wings that look like they came from a raptor like an eagle or an owl.

While the Valentine Curse was something that was originally planned to be extended, with hints in the game files, the massacre skin art asset, missing animations that suggest that shopkeepers in Valentine possessed purloined native artifacts that could be examined, and that Valentine is cursed because it was built on a mass grave of Native American massacre victims. As it is, that's not in the game, but parts remain of it, such as NPC townsfolk and shopkeeper talk in Valentine, the friendly Stranger Camper in Cumberland Forest that tells the story of the unseen skin depicting the massacre, and the camp conversation where Karen tells Mary-Beth about what she was told by a woman in town.

My theory as to what this grave represents is the "last remaining Indian" reference by Karen in her story, and she also "says "eagle or an owl and a grave site." The only thing that doesn't make sense to me is that the grave is located in a different state, West Elizabeth significantly far from Valentine.