Onderon was a planet which, despite centuries of settlement by an advanced space-faring civilization, never mastered the skills of exterminating local flora and fauna. For this reason the civilization consisted of one city, called Iziz.
A bunch of Jedi and Sith wound up fighting on Onderon for some reason. Arca Jeth, Nomi Sunrider, Ulic Qel-Droma, Cay Qel-Droma, Freedon Nadd and King Ommin all fought there. Decades later, Canderous Ordo and the Jedi Exile wrecked the place with a Basilisk war droid.