Tensei Shitara Slime Datta Ken: Bangai-hen - Toaru Kyuuka no Sugoshikata

Synopsis

Rimuru and his friends go to the "Magic Tower", a place where magic from all over the world gathers to develop new special moves, and they are supposed to be busy with their research, but they get caught up in a conflict... (Source: Official Website, translated) Note: Includes one extra chapter.