The Gazelle the king of Dwargon ventured to Tempest to officially seek to form a non-aggression treaty with Tempest. Also as an under the table deal, he wanted to form mutual technology research treaty, to share technological advancements. Essentially Dwargon was recognizing Tempest as an official state, a country with sovereign rights.
It was decided, the country shall be known as the Monsters’ City “Tempest.” They had intended to call it Rimuru, but that was too embarrassing so he forbid it. They comprised down to Tempest.