The Thunder Below

or will offer either this quest or The Crumbling Hall at  with the Golden Lotus when the Ruins of Guo-Lai is the third quest hub of daily quests.

Objectives
Slay Milau.
 * slain

Description
Roars have been echoing out from inside the Guo-Lai Halls. The mogu have awakened some ancient beast, an it is not long before they attempt to release it upon my men.

Find your way to the Hall of the Serpent and slay the beast before it can be unleashed upon us. I would trust no other with such a dangerous and important task.

Rewards
You will receive:

Progress
Have you slain the beast?

Completion
You accomplish what I doubt I could even ocercome myself. I am humbled by your valor.

Gains

 * 294000 XP
 * +200 reputation with Golden Lotus

Patch 5.4
The mogu have invaded the Vale in even greater numbers. This has resulted in many mogu mobs being located where once there were none (including Milau's room). Also, prior to 5.4, the Lightning Pool ability was infamous for rarely targeting where the player actually stood. This is no longer the case.

Because of the above changes, it's much more important now to bring along a friend if you are working towards the achievement.