There exist three realms; The Physical Realm, The Spirit Realm, and The Underworld.
The physical realm is the anchor to which all other beings in other realms traverse.
In the beginning, according to the record of mages, the physical realm was said to have once been ruled by mages who had once attained an untouchable level. The spirit realm was ruled by the so-called spirit king who also possessed unprecedented might.
Lastly, the underworld was said to have been ruled over by beings referred to as demons, and the demon lords were the all-time sovereigns of the realm.
Mages had the privilege of summoning the creatures of the spirit realm and the demon realm. But it comes at a cost.
The cost for a mage summoning a spiritual entity requires a high affinity for the summoning and a substantial amount of mana to carry out the summon.