The Tuterians were a spacefaring humanoid species from the Delta Quadrant. It is known that they had close ties with the Krenim Imperium for several years, which were still standing in the early 25th century. During the 2390s, the Borg threatened the Tuterian homeworld. To prevent themselves from being assimilated, the Tuterians used modified Solanae technology, successfully saving their home planet.
During the Vaadwaur Supremacy's attack on the Krenim Imperium, at least one Tuterian was on Kyana Prime when the Krenim used temporal technology to hide their cities by taking them out of the timestream.
When the Iconian Resistance used the Krenim temporal weapon ship to prevent the USS Yamato from discovering Iconia, it ended up creating a timeline where the Borg discovered the Tuterians and assimilated them, granting them superior tech used to invade and assimilate Romulus. In desperation, Captain Nog used the weapon ship to attack a Borg transwarp gate to fix the timeline, however, in doing so caused the timeline to change where the Tuterians were, instead, seemingly assimilated by the Borg before they could master the use of Solanae tech to defend themselves. (STO mission: "Butterfly")
However, a Krenim known as Noye discovered that the Tuterians had survived, escaping into another plane of existence that they can't escape out of, becoming the Sphere Builders. (STO mission: "Time and Tide")