The Assiti were a sophisticated and curious alien race that mastered spacetime, which they used in making "art". However, their amusement and practice in their "artwork" was dangerous. For example, a minor mistake by the Assiti caused the transportation of the West Virginian town of Grantville and its inhabitants to the midst of 17th-century Europe during the Thirty Years' War, creating a whole new universe. Several of the Assiti Shards have hit Earth, resulting in multiple temporal transpositions and divergent timelines.
Some 85 million years later, after many stern warnings against their "art", the Assiti were exterminated by the Fta Tei, a species descended from Humanity.