Monster Strike: Zoku Saishuu-banashi - Pandora no Hako

After defeating the War Gods, Ren and his friends are living the dream.
It seems like they've finally managed to regain their normal lives. However, a mysterious girl by the name of Pandora appears, bringing with her a box that must never be opened.
An unfortunate incident results in the box opening (of course), setting the world up for some big changes...

(Source: YouTube, edited)