Servant of N'Zoth Title
Servant of N'Zoth is a kind of an open-world PvP achievement. You need to become an Assassin by killing 10 players of the opposite faction, after that you will be offered a new quest. You will have to kill 10 players of your own faction without dying. You will become hostile for players of both factions. Overgear can lend you a hand with this challenging achievement. Let us know in chat if you are interested in it. As a reward, you will get a title <Servant of N'Zoth> and a toy N'lyeth, Sliver of N'Zoth.
There are not so many achievements in WoW about open-world PvP and even less that can bring you additional rewards. Servant of N'Zoth has it all. Feats of Strength achievement, title, and the toy. By the way, N'lyeth, Sliver of N'Zoth will allow you to become a Servant of N'Zoth anytime you want if you are in BfA locations, and this toy is not on CD!