A Delta Quadrant people whose scientist Annorax used a temporal weapon ship to erase entire species from history in a bid to restore the Krenim Imperium.
The Krenim are a humanoid species of the Delta Quadrant who rule the Krenim Imperium. They are distinguished by their mastery of temporal science, including chroniton torpedoes that can pass through ordinary shields because they exist slightly out of phase with time.
In the two-part story Year of Hell, set in 2374, USS Voyager enters Krenim space just as a scientist named Annorax is using a vast weapon ship to make temporal incursions, erasing whole civilisations from the timeline to reshape history in the Imperium's favour. Each change alters Voyager's situation, and for months the crew endure attacks that leave the ship barely holding together. Annorax has spent two centuries trying to restore the timeline in which his wife died, as an earlier incursion went wrong. Kathryn Janeway rams the weapon ship, and its destruction resets the timeline so that the year never happens.
The Krenim also appear in Before and After, and the species has become shorthand for the dangers of tampering with time.