Mana Contract.
A Mana Contract is a binding agreement made by a mana user, tying a fragment of their essence directly to their mana core. In exchange for fulfilling certain conditions, the user gains a significant boost in power. The mana core, located near the heart, serves as the central source of their energy and is intricately linked to their life force. Any violation of the contract's terms carries dire consequences, often affecting the user's mana, health, or even their life.
Mana Contracts remain an enigma to the world, their existence deliberately kept secret by the Four Great Kings and the Sovereigns. Apart from these rulers and a handful of apostles or select others, no one should possess knowledge of Mana Contracts—or the method to forge one.
Unless…
That individual had stumbled upon a certain book in another world and spent a year secretly mastering the art of forming such a contract.