Back when Izroth's party first arrived at the Settlement of Clearwater, they had a meeting with the members of Blue Oasis before the raid began. During this meeting, Guan Yu and one of the Captains of Blue Oasis, Minus, exchanged some unfriendly words with one another.
In order to save face for Blue Oasis, as well as the guild leader Asgard without offending Izroth and his party before the raid even started, Niflheim sent Izroth a private message. In return for dismissing the matter, Niflheim offered to give Izroth the method of acquiring territory in RML!
Of course, Niflheim never expected that things would end the way they did in the Great Sea Palace—especially with Sleeping Gardenia walking away as the ultimate victors. Nevertheless, a promise was a promise.