I love these guys for all the build up that is made around their appearance and they way they act. Mind you, the first 2 parts do not show the river tribe directly, we only see other the aftermath of their barbaric ambush (think stuff like the Viking blood eagle) or them strangely sparring people and just knocking them out.   
It really help that there doesn’t seem to be any pattern to their actions, making them seem beyond human in the way they act. Mind you they are also normal humans in this story, they’re the one calling the tribe folk spirit or ghosts due to their strange incomprehensible ways. Now granted at first you the reader like our heroes (the orcs and goblins) will doubt these claim. It could just be a tribe of your average cliché half naked savage…and then we see them for the first time and as you can see, they look straight up like mummified corpses, undead clad in rags, bones and dead leaves wielding primitive tools from the bygone Stone Age. Are they undead, or they not even human that is all you can wonder until our story progress further and you and the character realize that in truth the river tribes and their seer are probably more confused and afraid then actively malicious. We don’t know exactly what happen to them to make them this aggressive agaisnt outsiders (some characters have theories but none are confirmed due to the language barrier) too long autarcy, disease, poisonous magic, pollution? 

Who knows, but what is made clear is that the tribe actions are decided on a game of dice rolls, yes dice rolls, that are thrown by the seer who then interpret the dice face to command the tribe actions. And so when the seer is finally killed and the sacred dice are taken, the tribe is lost, too stunned to act, their fate now in the hand of their enemies, our protagonist, who later on visit the camp for parler , and as you can see in the final picture, under all their costumes and warpaint, the tribes is human, like you and me. Lost? Confuse? Yes, but human nonetheless. But you know what I ain’t gonna spoil all the finale for you and how la meute deal with them.  
All this to say that I genuinely love them as a deconstruction: going from undead looking monster, to frighten isolated humans.

The river tribes “spirit” Orcs et Gobelins : 18.la meute