The episode bundled with the 13th volume is about the Sumo competition that held at Tempest for the first time, suggested by Rimuru. The three-part OVA bundled with the 14th, 15th, and 16th manga volumes is an original trilogy written by Fuse. Rimuru has been teaching Shizue's students at Ingracia Kingdom's Freedom Academy. The time has come for the school's annual outdoor training event, where the students will test their combat skills in the field. Rimuru is determined to win the competition and take home the prize money, but he is challenged by Jeff, an honorary teacher who sees Rimuru as a rival. The competition is underway when suddenly an unexpected enemy appears!

Fifteen years ago, disaster struck human civilization, and now dangerous man-eating monsters roam the ravaged lands, posing an existential threat to the remaining survivors. Amid this turmoil, an isolated facility shelters children and nurtures them in peace. However, as a few among them find out about the world beyond the narrow periphery of their nursery's walls, their curiosity about it slowly grows. Meanwhile, in the outside world, young survivors Maru and Kiruko band together to search for a special place called Heaven, each for their own reasons. Carrying past burdens and tragic secrets, the two hope to find answers to the cruelty they have experienced in their lives and in the world, which still remains in tatters.

In the future, the "Metal Fight" games are the most popular televised sport in the world and many robot contestants compete for the title of Best Metal Fighter in the World. Kurenai Ken, along with teammate Saeba Shoko (who was, at first, adamant), pilots the Super Robot Fighter, Red Baron and enters the contest with dreams of becoming its champion. He must, however, face an army of other rivals from around the world including Kaizer (who is, in fact, Shoko's supposedly "lost" father, Tetsuo Saeba), the Tetsumen Tou ("Iron Fist" - a reference to the original Red Baron TV series) doctors, Tiger and ShinRon.

Mini anime for Munou na Nana released weekly on the official Twitter and website.