A Delta Quadrant species who, together with the Mawasi, provide ships for Janeway's attack on the Krenim weapon ship in the second part of Year of Hell.
The Nihydron appear in the Voyager two-part story Year of Hell. Their space lies in the region shaped by the Krenim temporal incursions, and like their neighbours they have reason to fear a vessel that can remove whole civilisations from history.
In the altered 2374, with Voyager barely holding together, Janeway reaches an understanding with the Nihydron and the Mawasi. Their ships are equipped with temporal shields developed by her crew, and they join her in a co-ordinated assault intended to disable the weapon ship and rescue Chakotay and Paris, who are held aboard it.
The attack costs the allies dearly, but Paris's mutiny among the Krenim crew drops the ship's own temporal protection at the critical moment. Its destruction restores the original timeline, in which none of these events occurred.