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

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.*