Nygeans were distinguished by a ridge that went from the top of their foreheads down to their necks. They also had larger ridges on their noses and chins. Their skin was blotched in appearance, and they had pale spots going down the sides of their faces.
The Nygeans governed a sector containing several humanoid species, one of which was the Benkarans. The Benkarans, who occupied less than 10% of Nygean space, made up more than 80% of the prisoners in Nygean prisons.
Nygean ships were capable of warp factor 8.2.
The Nygean legal system was based on an ancient protocol known as "Vekto Valek K'vadim" or "favor the victims", in which the victim and/or their family determined the sentence for the offender, from restitution up to and including the death sentence.
The Federation's first encounter with this race took place in 2377, when a prison ship was rescued by the USS Voyager. Its (surviving) complement was comprised of three Nygean guards, three Nygean prisoners, and five Benkaran prisoners. (VOY: "Repentance")
When Voyager was pulled into the Void, they encountered another Nygean ship under the command of Captain Garon. The Nygean ship, which had been trapped in the Void for a long time, was the first to enter into an alliance with Captain Janeway.
In a joint effort, they discovered that a polaron modulator would be able to create a shield bubble large enough to encompass all the Alliance ships, and the Nygeans were a great help in the construction of one. When Bosaal was asked to leave the Alliance, he took several ships with him, but the Nygeans decided to stay. Within days, the Nygean ship and all Alliance members had escaped the Void. The Nygeans said goodbye to Captain Janeway, since they were heading in the opposite direction, and wished Voyager's crew luck on their journey home. (VOY: "The Void")