Cohlien Frostweaver is a male gnome mage. He's also one of the four spirits that can be put to rest in Kirin'Var Village. He died when Kael'Thas forces attacked the village.
The item that binds him to the village is his "lucky" hat:
"Looking at the hat, you find yourself asking if any Kirin Tor mage would own such headgear. Judging from the hat's wear, it seems the gnome wouldn't go anywhere without it.
You try on the hat momentarily, discovering no magical properties beyond a faint urge to dance.
The treasured possession might be the source of the dead mage's bond to the village. Placing the hat in the footlocker in Cohlien's quarters might free his spirit.[1]"
Returning the hat will trigger a cutscene and will make Cohlien neutral towards you.