The Headhunter's Trophy is an exclusive item obtainable by completing the "Headhunter" achievement. The achievement requires completing 5 bounty quests.
Additional trophies can be earned for every 5 bounty quests completed: after completing 5 quests, you will earn another for your 10th quest, and another for your 15th, and so on.
If you combine the Headhunter's Trophy with a weapon, it will get the "Headhunting" modifier. The name explains it all: It allows the wielder to collect heads from their fallen enemies.
The Headhunter's Trophy can not be added to weapons with any sort of "bane" modifier.