How I Stopped Worrying and Learned to Love Eberron
This magical race is often mistaken for ogres, but they are not the same species. The two share similar physical features, but they are two offshoots from a common origin. They also share a common ancestor with trolls and orcs. While ogre blood emphasizes physical ability (with a few exceptions like ogre mages), oni blood is more magically inclined, with a concentration on abilities like shapeshifting.
Oni are more common in Sarlona, particularly in Syrkarn, but also in the Horned Guard of Riedra. They are believed to have originated in what is now the Riedran province of Borunan, where an ogre and oni military culture ruled a series of warring city-states.