When Souichi wakes up, he finds himself turned into a demon king for some reason. In order to prepare for a war between demon lords, it is said that you must strengthen the defense of your dungeon. However, the skill given to Souichi was "only the weakest rank monsters can be summoned". With this, he can't win... Souichi despairs, but his skill actually has great potential. Make full use of the summoned monsters and aim for the strongest demon king with "gacha" and "combination"!