Arkhos is a plane in which day and night intermingle, creating a twilight in which one never knows what is dream and what is reality.
Connection with Theros
Mark Rosewater has stated that the planes showcased in Planechase and Planechase 2012 were created as potential locations for future stories. However, the Greek-themed Arkhos has been rebranded to Theros, just as Mongseng was rebranded to Tarkir. Rosewater has stated that bigger factors than continuity led to the name change. He later stated that they couldn't clear the Arkhos name. According to Doug Beyer, both planes now exist in the multiverse.