Energy beings living aboard a failing ship, who trade places with members of Archer's crew as a prelude to taking their bodies permanently in the Enterprise episode The Crossing.
The Wisps appear in the Enterprise episode The Crossing. They are a non-corporeal species who exist as drifting lights and travel in an enormous vessel that swallows Enterprise whole.
At first the contact looks harmless. A Wisp can exchange places with a humanoid, letting the visitor try out physical life while the displaced mind roams free, and Tucker returns from the experience delighted. More and more of the crew are then taken over, and T'Pol learns through telepathic contact that the aliens' ship is breaking down. They intend to keep the bodies they have borrowed.
The unaffected crew shelter in the catwalk of a warp nacelle, which the Wisps cannot enter. Phlox floods the rest of the ship with carbon dioxide to render the hosts unconscious and drive the aliens out. Enterprise breaks free and destroys the alien vessel.