A time loop that Zorian, Zach, and possibly Red Robe are trapped within.
Every month time seems to go back to the start of the month. Zach was originally in the loop, likely due to a marker on his soul. Quatch-Ichl used soul meld on him and Zorian, transferring a chunk of Zach's soul to Zorian along with the marker. This caused Zorian to loop as well.
Zorian's soul and memories are looped but not his physical body, so he can improve his skills, memories, mana and spells using the loop but can't become much fitter. Any injuries other than ones to the mind or soul are removed at the end of the month.
Zach's death seems to end the loop, as does the emergence of a primordial. Astronomy seems to be normal, and the spiritual realms (angels, demons, elementals) are cut off.
The soul marker on Zorian and Zach's soul can be tracked, and has a switch to end the loop.