(covers information from an alternate timeline)
In one possible timeline in 2374, three Nihydron warships joined the USS Voyager in its attack on the Krenim weapon ship. B'Elanna Torres and Harry Kim were assigned to work with the Nihydron to implement temporal shielding aboard their ships. (VOY: "Year of Hell, Part II")
The current state of the Nihydron Empire is unknown, after the temporal weapon ship was erased from time, undoing all the damage it had done. Species 10026 ships shown in "Dark Frontier" are identical to the Nihydron ships shown in "Year of Hell".
The Nihydron reappeared in the 2016 novel A Pocket Full of Lies. The novel depicted a second iteration of Voyager's first contact with the species, since the events of the Year of Hell – where Voyager made first contact with them in 2374 – were erased from history.