Monster Strike: Zoku Saishuu-banashi - Pandora no Hako
English name: An Encore and Continuance: Pandora's Box - A strange phenomenon is overtaking the world?! -
Genres: Action, Fantasy
Type: ONA, Episodes: 1
Aired: Dec 31, 2016
Studio: Studio Hibari
Source: Game
Rating: PG-13 - Teens 13 or older
Synopsis:
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)