Evocation is when a summoner uses external magic power to summon or provide a portal for another entity to appear before them. Evoking is usually performed through wards, incantations, lyrics, components or through a focus.
Invocation is when a summoner uses their own magic power to become a conduit for a spiritual entity who can act through the summoner or provide their power to them. Due to its nature, invocation only works on beings with a corporeal body and only with spirits.
Exorcism is essentially the opposite of invocation and forces a spirit to leave a body.
To strengthen the bonds a summoner has with the spirit(s) they invoke, he/she should consider making a official agreement known as a pact. Pacts involve a mage linking their internal magic power with a spirit which allows them to use a spirit’s sorcery when invoking them. Conversely it also allows the spirit to use the sorcery of the summoner’s race or inhabiting and acting through their body during the invocation. Another benefit of a pact is the creation of a mental link that lets one know when the other is in mortal danger, regardless of distance. Also if a spirit takes control of a summoner during an invocation, the link of a pact allows the summoner to remain fully aware.
Invocations without an official pact, merely has the spirit possess the summoner until the mage’s magic power is depleted and doesn’t remember anything during the ordeal.
In order to access each other’s power, both parties have to offer a price in exchange. A spirits price is usually just being on call and granting their power while the summoner’s
price can range the mage providing the summoned with food, getting them a certain object, being temporarily possessed by the being whenever it wills it, to becoming a servant or lover of the summoned being.