Short answers, pulled from the story.
Evocatio was a calculated religious maneuver used during sieges to divert the tutelary deity of a besieged city away from its defenders and toward the Roman army. Roman commanders offered the enemy god a more lavish temple and a richer cult in Rome if they surrendered. This ritual allowed the Romans to claim divine favor without committing sacrilege.
Neoplatonism and theurgy introduced a system where the calling forth of spirits was a method to elevate the human soul toward the divine. Practitioners believed that through specific rituals, they could communicate with higher powers and gain knowledge that was otherwise inaccessible to the mortal mind. This tradition moved away from the coercive nature of Roman warfare and embraced a more mystical approach to interacting with supernatural agents.
The Greater Key of Solomon the King and the Lesser Key of Solomon, also known as the Lemegeton, became the primary texts for those seeking to command spirits. The Sacred Magic of Abramelin the Mage offered a different approach, focusing on the attainment of the knowledge and conversation of the Holy Guardian Angel. These grimoires provided detailed instructions on how to construct magical circles, prepare tools, and recite the correct formulas to summon demons and angels.
John Dee, a renowned mathematician and advisor to Queen Elizabeth I, began a series of evocations with Edward Kelley, a self-proclaimed medium. Together, they claimed to have summoned spirits who revealed ancient secrets and prophecies about the future of England. Their work was documented in detailed diaries, which described the rituals and the entities they encountered.
Groups such as the Hermetic Order of the Golden Dawn and later the Thelema movement adopted and adapted the rituals described in the grimoires. These modern practitioners sought to revive the ancient arts of summoning, blending them with contemporary philosophical and psychological ideas. Hoodoo and other folk magic traditions also incorporated elements of conjuration, using incantations and charms to cast magical spells.
The use of mind-altering substances, whether herbal or chemical, can alter the practitioner's state of consciousness, making them more susceptible to the experience of contact with supernatural agents. This altered state can lead to vivid visions and a sense of connection with the divine or the demonic. The use of uttered word formulas and the repetition of specific phrases can induce a trance-like state, facilitating the experience of summoning.