Short answers, pulled from the story.
Evocation is the act of calling upon or summoning a spirit, demon, deity, or other supernatural agent within the Western mystery tradition. Practitioners use rituals, incantations, charms, or in some traditions mind-altering substances to make contact with invisible powers.
Evocatio was a Roman military ritual that summoned a city's tutelary deity away from an enemy and toward Rome, either during a siege or after surrender. Rome typically promised the deity a better-endowed cult or a more lavish temple in exchange for switching allegiances.
Necromancy is a specific form of evocation focused on summoning the ghosts or spirits of the dead for the purpose of divination. General evocation may target demons, deities, or any supernatural agent, while necromancy is defined by its focus on deceased spirits and its goal of obtaining hidden knowledge.
The most prominent grimoires associated with evocation are the Greater Key of Solomon the King, The Lesser Key of Solomon (also known as the Lemegeton), and the Sacred Magic of Abramelin the Mage. These texts provide structured procedures for summoning and communicating with spiritual entities.
Conjuration and evocation overlap substantially, both referring to the invocation of spirits or supernatural powers through magical acts. Conjuration is also used in traditions such as Neopagan witchcraft, hoodoo, and Hermeticism to describe casting spells by a variety of techniques beyond summoning alone.
Calling forth spirits was a relatively common practice in Neoplatonism and theurgy in antiquity, where spirit contact was treated as a legitimate philosophical and spiritual pursuit. These ancient systems fed directly into the Western esoteric tradition and the grimoire literature that followed.