Nuada was a general working for the Tuatha de Danaan.
The general was sent to ask Goibhnie to save Tír na n-Óg, but the Troifran scientist did not want to be disturbed. He sent out a horde of demons, who Nuada said destroyed most of the accompanying troops.
In fact, Nuada was killed and impersonated by a shapeshifter, who then helped his fellow demons evacuate to Earth. When Goibhnie found out and tried to stop him, Nuada changed into his demonic form and killed him. Then the infected David involuntarily transformed into a demon and killed the imitation Nuada. (PROSE: Cat's Cradle: Witch Mark)