This is story related to "Tiny Tina's Assault on Dragon Keep." Its unlocked more or less upon the completion of the DLC when you also more or less
Its pretty self explanatory but, here's a walkthrough of the whole DLC courtesy of IGN.
https://www.ign.com/wikis/borderlands-2/Tiny_Tina%27s_Assaul...As a side note when I ran this DLC on both the original edition of Borderlands 2 on the Xbox 360 and on the Xbox One "Handsome Collection" version, both times I was playing as Gaige the Mechromancer solo with a focus on her "Best Friends Forever" and "Little Big Trouble" skill trees versus the meta of using "Ordered Chaos."