The Seal is Broken is a quest in World of Warcraft's new limited-time mode, WoW Remix: Mists of Pandaria. This mode allows you to level a new character through Pandaria but with a few twists along the way. Think of it as WoW retail's version of Season of Discovery, but available for a much shorter time.
If you levelled through the original Mists of Pandaria expansion when it first launched, or have even levelled other characters, you'll probably remember many of the quests along the way.