The Turei are a space-faring culture from the Delta Quadrant. They are the current keepers of the underspace corridors that traverse the Delta Quadrant. This underspace was once the property of the Vaadwaur, but in 1484 the Turei led an alliance of the Vaadwaur's enemies, launching a massive orbital assault on the Vaadwaur homeworld.
With the Vaadwaur apparently defeated, the Turei maintained their control over the underspace until 2376 when the USS Voyager accidentally entered it. Unable to escape from it, Voyager passed a Turei starship which forced Voyager back in to normal space. They then demanded that they be allowed to board Voyager in order to remove all data on the underspace. When Captain Janeway refused, the Turei pursued Voyager to the Vaadwaur homeworld. Voyager entered the planet's atmosphere, which was still irradiated centuries after the bombardment.
Voyager's crew discovered and revived those Vaadwaur who had entered in to stasis prior to the Turei attack. These Vaadwaur quickly proved themselves to be treacherous, and attempted to take Voyager. However, with help from the Turei, Voyager was able to escape. (VOY: "Dragon's Teeth")