(covers information from several alternate timelines)
The Rilnar were a species native to the Delta Quadrant. Mortal enemies of the Krenim, the Rilnar were what originally inspired Annorax to create the Krenim weapon ship, with which he erased the Rilnar from history in the 2170s.
However, although the subsequent alterations to the timeline restored the Krenim Imperium to its former glory, it also removed a crucial antibody the Rilnar had contributed to the Krenim genome. As a result, fifty million Krenim died in the first year after the restoration including Annorax's wife. The Rilnar were restored to their original state when the weapons ship itself was erased from history. (VOY: "Year of Hell, Part II")