Warning is solid attachment card, very useful, mainly while using a heroes with lower value. I usually use two copies in deck, which is good enough to boost one defender. I also use moving action with this card much often then in the case of Dunedain Mark. Only the artwork could be a little bit better. Verdict: 4/5.
Attach to a hero.
Attached hero gains +1
Action: Pay 1 resource from attached hero's pool to attach Dúnedain Warning to another hero.
Conflict at the Carrock #26. Leadership.
Dunedain Warning is a nice, good, vanilla card. Extra defense is never bad (except on maybe Gimli?) and this one is non-restricted, costs one in the richest sphere and has that extra versatility if the encounter phase goes awry. One of my favorite uses is in a resource gen. deck where in the late game, it often ends up top-decking, so the money piles up. I can pass this around to every hero I need to defend and know they'll be in ship-shape.
(its interesting: finding 200 words to describe the Dunedain Warning isn't easy.) no one would question its inclusion in a mono leadership deck, also good for multiplayer. 4 stars in the Dunedain sky.