Shado-Pan Geyser Gun

The Shado-Pan Geyser Gun is a lesser known item acquired by a very unusual means while in the Throne of Thunder. To acquire the item players need to use the geysers in the Lair of Tortos and bounce from geyser to geyser in a circle 250 times. Each geyser has its own cooldown, so bouncing back and forth will not work.

At level 91 or above, speak with Lorewalker Han at the Seat of Knowledge in the Vale of Eternal Blossoms to queue for the Forgotten Depths of the Throne of Thunder in Raid Finder mode. Players will spawn in the area with the geysers.

Source
When you reach 250 stacks you'll receive the item in the mail with the following message: