Kelvan

"We do not colonize. We conquer. We rule. There's no other way for us."

- Leader of the Kelvan expedition to the Milky Way Galaxy

The Kelvans are an advanced civilization native to the Andromeda Galaxy, which has power over very sophisticated forms of technology.

Their original form is oddly shaped, containing hundreds of tentacle limbs, and have none of the emotional concepts shared by most humanoids. Kelvans are capable of constructing spaceships that travel the distance between close galaxies in only 300 years – far more rapid than any Federation vessel was capable of in the 23rd century. This, however, is longer than the life span of a Kelvan, so that intergalactic ships are crewed by multiple consecutive generations who live aboard.

The Kelvans believe increased radiation levels emanated in their home galaxy of Andromeda will render it uninhabitable for them in about ten million years. They have sent a number of multi-generation vessels to the nearby galaxies to find one suitable for invasion. Their philosophy is seemingly devoid of the concept of pacific coexistence with alien species, as they readily dismiss the possibility of colonization of uninhabited planets and prefer to forcefully conquer other species' territories instead, making them an irrationally xenophobic race.

History
In the 23rd century, the last five Kelvans from one of their expeditions arrived in the Milky Way Galaxy, after having their main vessel unexpectedly destroyed by crossing the Galactic barrier, and assumed the form of Humans to better adapt to the technology of the natives, which they expected to take and modify for the trip back home. They deemed the galaxy an adequate place for conquer and established the intention to hijack the USS Enterprise, and modify it to return to Andromeda to inform and prepare their kind for an invasion in full scale. However, their ability to assume a Human form proved more problematic than they thought, as they started to behave like Humans and experience such new concepts as vice, jealously and sexual attraction. The Enterprise crew took notice of this and easily manipulated the invaders into fighting among themselves. While the Human emotions caused them anger and frustration, they ultimately had a beneficial effect on the Kelvans, making them more receptive to the Federation offer of peaceful coexistence in the Milky Way. As the Kelvans returned the Enterprise command and took the ship back into the galaxy, they also began to make plans for colonization of an uninhabited planet and sending a robotic probe to inform those in Andromeda of the proposed solution.

Behind the scenes
The Kelvans in Andromeda are expected to receive the message of peace somewhere in the 26th century, and arrive in the Mily Way in the 29th century should they indeed decide to depart. Those left in the Milky Way might or not have decided to join the Federation, and perhaps even assume full-time status as humans, since they appear to have come to enjoy the new sensations and way of life. However, the only canonical mention of Kelvans after the incidents of "By Any Other Name" is made in Deep Space Nine, when Worf mentions he once fought Kelvans twice his size. The mention indicates that at least some Kelvans maintain, or have returned to, their original shapes, and have engaged in conflict with the Federation and/or the Klingon Empire.