Barrow Wights, those bastards. Always possessing Rangers and turning them against their former brothers. When investigating the barrows, be on the look out for the telltale signs that your friends aren't entirely of their own mind. Are their eyes glowing green? Is there a monstrous apparition lurking just over their shoulder? Have they suddenly started arguing that Return of the King is the best of the trilogy when it's clearly Fellowship of the Ring? If the answer to any of these is yes, stab first and ask questions later.
The trailer up there for co-op action RPG Lord of the Rings: War in the North shows us exactly how those undead cretins should be dealt with. Ie. a stern telling off followed by a firm ass kicking using one or all of the ranger, melee and magical variants that make up War in the North's mini-fellowship. All three are playable in co-op, too, so you'll be able to lunge into every combat shouting "YOU HAVE MY AXE" with every slash until your friends disown you forever. War in the North is out on November 1 in the US, November 25 in Europe and sometime towards the end of The Third Age in Middle Earth.