There are quite a few achievements that you need the Faceless Masks to obtain but the two most notable one are
幻象彼岸 and
癫狂世界. Both of these achievements have a reward that is more than just achievement points:
- 幻象彼岸 requires a 5 mask run in both Stormwind and Orgrimmar, in addition to some other requirements. This rewards the 恩佐斯的黑蟒 mount.
- 癫狂世界 requires a solo 5 mask run in both Stormwind and Orgrimmar. This rewards the title.
The black serpents of N'Zoth are the harbingers of his ultimate vision becoming reality. Only those servants or enemies that can tread this space have any chance of wrangling them.
Achievement:
幻象彼岸