It's available in the Store and can be placed inside houses and guildhouses. Use it to fill all empty vials (of any size) inside your backpack and turn them into Supreme Health Potions. The cask can be used to fill 1000 potions and disappears when empty.
Note: Potions filled using casks will be placed inside Your Store Inbox and will have the same restrictions as those bought from the Store, i.e. they cannot be traded, put on the ground or sold in the Market, but can be stored inside your depot.
See also: Cask & Keg Value Effectiveness