Rimuru is building a dungeon in preparation for the upcoming festival. It will feature treasure chests containing weapons made by Kurobe, and Veldora will inhabit the deepest floor as the final boss. Then Milim suddenly shows up with a suggestion...
An arena above ground, 
and a dungeon underground.
حلبة فوق الأرض، وديماس تحت الأرض
Überirdisch eine Arena,
unterirdisch ein Dungeon.
Sopra ci sarà l'arena,
sottoterra invece il dungeon.
Uma arena na superfície
e uma masmorra subterrânea.
На земле — арена,
а внизу — подземелье!