It's appearance is that of a hollowed out tusk adorned with decorative steel casings - the motif being similar to that found on steel weaponry.
The horn is obtained from Elisif the Fair after you complete the quest The Man Who Cried Wolf, alternatively it can be pickpocketed from her. She asks you to take the horn and place it upon the Shrine of Talos, located north east of Whiterun, at the base of a small mountain to the west of Fellglow Keep.
Upon completion of the quest, the horn is no longer marked as a quest item so it may freely be picked up and dropped.
Although there are several locations for the Shrine of Talos, the horn may only be dropped at the location above, northeast of Whiterun.(PS3- The location is generated randomly-confirmed)