Dunedain Signal is a very good card. For one resource you can have a defender for the entire table. Of course, many characters that you want to defend already have sentinel, but there will nearly always be one that doesn't. The other cards that grant sentinel are Shadowfax; which only attaches to Gandalf, Elven Mail; which only attaches to Noldor or Silvan; and Arwen; which is where most of the Signal's competition lies. What it comes down to is whether or not you want to pay an extra resource for two willpower. One important note for all sentinel cards is that... sentinel is useless in solo!
Attachment. Cost: 1.
Attach to a hero.
Attached hero gains sentinel.
Action: Pay 1 resource from attached hero's pool to attach Dúnedain Signal to another hero.
Return to Mirkwood #117. Leadership.