Powerful beings from the Andromeda Galaxy who took human form, seized the Enterprise and planned to sail it home before discovering the cost of human senses.
The Kelvans are natives of the Andromeda Galaxy, where their empire faced a slow catastrophe as rising radiation levels threatened to make their home uninhabitable. They sent expeditions to the Milky Way in search of new worlds for their people.
In their natural state the Kelvans are immense beings with a great many limbs, but to operate in human environments they adopted humanoid bodies. They are highly disciplined and devoted to their mission, and wield technology able to paralyze people with a neural field or reduce them to small cuboctahedron blocks.
In By Any Other Name, a Kelvan group led by Rojan took control of the Enterprise to make a centuries-long voyage back to Andromeda. Kirk's crew turned the Kelvans' new bodies against them: Scotty drank Tomar into a stupor, McCoy agitated Hanar with stimulant injections and Kirk stirred jealousy over Kelinda. Overwhelmed by unfamiliar feelings, Rojan agreed to settle on a planet the Federation would provide instead.