Thrain was settled by humanity as part of a peace agreement with the Sun Elves and Dwarves. Thrain has served as mediators for many of the races in Hellan and later became a driving force for the foundation of the Confederacy.
Geopolitics
Humanity organized the Confederacy, which unites the Sun Elves and Dwarves. They aim to bring all of the races of Hellan into one big alliance against Isandor. Currently led by Markus Thrain, those wishes may come to fruition. Humans are scattered everywhere in Isandor and live peacefully in Sun Elf and Dwarven land.
Humans have more motivations driving them to peace than simple altruism. The human capital of Thrain was founded on land given to them by the Sun Elves and Dwarves. It lies in a precarious position. It is vulnerable to attack by Dark Elves and Dark Dwarves, as the Mountains of Thrain have been tunneled into by the Dark Dwarves. Outlying settlements south of the snowy mountains are also vulnerable to strikes without protection from Thrain, due to a poorly connected empire.
Some humans are allied with the Dark Elves and shun Thrain’s people. It is not an insignificant group. Despite being fully integrated into Dark Elf culture, with half-elf sons and daughters, these people are still Thrain’s kin. They are a strong motivator for humans to want to ally with Dark Elves. The Shadow Hunters were one of these efforts.
In recent times as of the Drums of War campaign, the Dark Elves have entered the fold of the Confederacy, and now call themselves "Star Elves". The Star Elves and their accompanying half-elf and human allies are being re-accepted back into Thrain and reunited with their kin.
Trivia
Bastards of noble lords are given the surname 'Thrain' to denote their lineage, and importance. They typically serve as ambassadors, because they are both important yet expendable.