What does Eowyn give to merry as a parting gift, and what will it do?

"Éowyn presented Merry with a parting gift: the Horn of Rohan, a small

silver horn taken from the hoard of Scatha the dragon. It was an ancient, dwarf-made horn of silver, engraved with horsemen riding in a line from the tip to the mouth. Runes were set into it that gave it the ability to cause enemies fear and friends joy, bringing the blower's friends to him."