Introduced with the Heavensward expansion for Final Fantasy XIV, up to 24 players can take part in this raid.
Once you've unlocked two Alliance Raids they'll be available in the daily Alliance Raid Roulette, along with any others you might unlock later.
Unlocking The Weeping City of Mhach FFXIV
In order to unlock the Alliance Raid, you will need to complete the quest called The Weeping City. This quest is available by talking to Stacia in Sea of Clouds (X: 6.2 Y: 5.8).
To unlock the The Weeping City of Mhach quest you will need to finish a few other quests.
You're good to go!
You're now able to access The Weeping City of Mhach Alliance Raid.