Prime Magus is a common enemy in Tartarus' Harabah Block, from floors 202 to 213. It may later be found in the second and third section of Antenora within the Abyss of Time. In battle, it may be supported by a Desirous Maya, which will use Ice Break to allow the Prime Magus to effectively strike all party members with its Ice attacks.
Prime Magus is a common enemy on Paradise 1-2 of Heaven. It will simply use its Fire elemental spells and Mind Charge boosted spells to attack the party. Once defeated, it may drop the Smart Clasp material. Selling 4 Smart Clasps will unlock the Courtesia ougi for Yukiko Amagi in the shop.
In Golden, it may also drop Rose Quartz for trade at Shiroku Pub.