Brumund wanted to form a cooperative relationship with Tempest. They propose to cooperate to support adventurers on their adventure and assist them when they’re having problems facing powerful monsters.
Brumund is a small country without much national power, their counter-measures against monsters are insufficient. They somehow cooperated with the Freedom Association, but the association can’t be their only means of defense against monsters. Fortunately, there aren’t any major damage caused by monsters to date, but in preparation for the future, they wished to form bilateral relations with Tempest. In order for the adventurers to succeed, and to reduce the threat the forest poses, Tempest would need to provide necessities such as beds and supplies for those working in the forest.
In turn Brumund would try to guarantee certain benefits there, and legitimize their identity as a nation. In other words, Brumund promised to recognize Tempest as a legitimate nation of monsters.
In addition they would mutually stay vigilant about monsters and help each other in times of emergencies. Rimuru saw this as either way, they are supposed to be on guard, but with this, they would not have to worry about Brumund. They will notify and help each other when faced with assaults by powerful monsters.
In the event Brumund had to move a force through the forest, they would be counting on Tempest’s cooperation. However, this wasn’t something related to monsters. Because, they are to cooperate during emergencies, that also includes when the nation is being attacked. From Brumund’s side, they now have a reason to get Tempest to cooperate even without their consent. Brumund feared the Eastern Empire would invade through the forest. In preparation for if that happened, they wanted Tempest to become the buffer state. However if Tempest had a crisis, they would receive aid. Even if the empire ignored Tempest, they would have to send in reinforcements to Brumund.