Phase-shifted aliens who travelled to 19th-century Earth to feed on the neural energy of dying humans.
The Devidians are serpent-like beings from Devidia II who exist slightly out of phase with normal space, making them invisible to most sensors. They feed on the neural energy released by dying lifeforms, and can disguise themselves as humans when they need to move among their prey.
Their activity came to light when Data's severed head was discovered in a cave beneath San Francisco, buried there for centuries. The Enterprise-D traced the anomaly to Devidia II, and an away team followed the Devidians back to 1890s San Francisco, where they were exploiting a cholera epidemic to harvest neural energy unnoticed.
There the crew met a younger Guinan and the writer Samuel Clemens. Picard and his officers eventually closed off the Devidians' time portal, ending their feeding on Earth's past. The two-part story bridged the fifth and sixth seasons of The Next Generation.