Arcera is a world referenced by Bhelliom in The Hidden City. Very little is said about Arcera. Presumably, it was created by Bhelliom and then stolen by Klæl, who then utilized the massive natives as an army to fight the Church Knights and their allies. According to Aphrael, the native creatures breathe with their livers as opposed to their lungs (which they probably lack entirely), and as such circulate bile through their bodies instead of blood. This, coupled with Aphrael's quip about not wanting to smell Arcera's air, implies that the atmosphere of Arcera may be rich in sulfur, acids, and/or other toxic compounds, and thus is uninhabitable by "conventional" life forms. (Likewise, the creatures native to Arcera seem to be unable to breath the air of Sparhawk's world for very long.) Khalad makes the observation that the atmosphere is likely similar to marsh gas or firedamp (though Aphrael points out that they are really the same thing), thus allowing Klæl's soldiers to easily recuperate in sealed caves.
While highly unlikely, given their seemingly peaceful nature and tolerance of pure air, the legless aliens encountered in The Shining Ones may also originate from Arcera.