The Book of Lyl
Though the term originally referred to any races besides humans, Invaders are today most commonly recognized as beings from other planes of existence, demons and devils, angels and archons, and all manner of elemental creatures. When they crossed the Veil into Terentius, they brought with them strange illnesses that the native population was not prepared for, leading to a centuries-long Plague spreading throughout the world.
Though not inherently evil, Invader races are looked at with great skepticism. Some humans have made peace with the visitors, leading to small collections of half-breed races, such as Tieflings and Aasimar.