Xa Nui is an island in the Matoran Universe, and is populated by Xa- and Av-Matoran.
The island was created as a part of the body of the Great Spirit Mata Nui.
The banished Great Being Antidax, working through Velnax, established a hidden base for the Order of Darkness underneath the island. The Great Being Brominax was imprisoned there. Under Velnax's orders, Makuta Kunaku released the Shadowdermis virus on the island and infected all the Matoran residents, to keep anyone from discovering the location of the base.
Fairon and Rantu were later diverted there by Shadowdermis' powers, and were ambushed by Kunaku and the infected Matoran. They defeated Kunaku and continued to Sacho Nui in their pursuit of Brominax, not realizing that he was hidden on the island they had just left.
Helryx was able to identify the location of the Order of Darkness base, and sent Fairon and Rantu to bring Brominax back. They were confronted by Velnax, who bested the two Toa and the weakened Great Being. Fairon, however, managed to destroy Shadowdermis' substance, which went up in flames and distracted Velnax long enough for Brominax to pin him down. Velnax escaped, and his hidden base was destroyed.
Due to the destruction of Shadowdermis, the infected Xa-Matoran were suddenly cured, and most returned to Xa Nui.
- Xa Nui was also one of ToaFairon's first islands.