This will have to be off the beaten path a little. Having them chase some game or something could easily lead them to this.
They go over a small rise that reveals a lone tree with silhouettes of people hanging off of it. Upon closer inspection they find three young men that are all naked and unconscious, a slight breathing in each of them can be detected. They are suspended by two ropes that are connected to a bone that is thrust through each of their breasts. Blood trickles from the wounds as they gently sway in the breeze.
If they release the men they will meet no resistance.
At some point some tattooed men with scars on their chests matching where the hanging men were hung by will arrive. Preferably not too long after they take the hanging men down. If the players didn’t release the men then the tattooed men roughly tell them to move on. If they did release them then the tattooed men get very angry and very violent and try to capture the players. Hopefully they will succeed.
When caught they find out that they will be held until the three men are fit to see them. The men were going through a ritual to become warriors. They will not take kindly to their endurance test being interrupted. Unless the party can think of a good way to redeem the men or escape, the men will most likely take their lives in a substitute ritual to make them warriors.