The Archein (är'kən) were a race of highly technologically advanced beings from the Andromeda Galaxy whose home world, Archeinia, was 2.5 million light-years from the Sol system. (Star Trek: Odyssey, Star Trek: Hidden Frontier)
The Archein were led by the Grand Majan. The Archein's faith focused on worshiping the Great Ones. One precept of their faith instructed followers not to take things by force from others.
In the late 24th century, a slowly expanding quantum singularity was a major threat to the Archein and the Kelvans. By this time, one of the Kelvan moons had been encompassed by the singularity. It was predicted that by 2394, the singularity would make the quadrant that the Archeins occupied uninhabitable. (Star Trek: Odyssey: "Iliad")
Infiltration of Milky Way Galaxy
In 2384, the Archein began a military conflict in the Milky Way Galaxy through the use of an artificial wormhole, in order to colonize from their home in the Andromeda Galaxy. This invasion was conducted under the military command of General Morrigu and the leadership of Majan Seram. The intent was to colonize in the Milky Way Galaxy due to a growing singularity which threatened the Archein's home planet and its entire region of space. During 2384, the Archein had captured at least two systems inside Romulan space.
Seram had deceived the leader of the Archein people and the Grand Majan, who was in poor health, to believe that the wormhole led the Archein to an uninhabited region of space inside the Milky Way Galaxy. In truth, it opened into occupied Romulan space.
Remnants of the Archein remained in the debris field of the battle that took place between the allied Alpha Quadrant species and the Archein. When the USS Intrepid answered a Romulan distress call in the field, members of its crew were found victim to an Archein prisoner-processing facility. When the crew was rescued, the facility was destroyed. (Star Trek: Intrepid / Star Trek: Hidden Frontier: "Orphans of War")