Invocation is the nearest "real world" equivalent to the summon magic usually found in RPGs.
It is generally neither as flexible nor as easy to use, but is frequently less of a challenge to the suspension of disbelief.
This style of working involves calling a supernatural entity into your workspace, usually under controlled conditions, for the purposes of bargaining with it for power, information or practical aid.
In an RPG this is generally high level magic, but in most magical traditions (and many a minatory legend) anyone who knows the right ritual (or something approximately right…) can get the target entity (or something similar) to pay them a visit. Doing so safely on the other hand usually requires extensive preparation and frequently mastery of binding magic and/or ward magic. This is what the traditional summoning pentacles, magic circles and what have you are meant for - although whether they provide a refuge for the wielder, a prison for the entity or merely a dangerous illusion of safety varies. These precautions may be part of invocation or may be a seperate school - perhaps abjuration or simply ward magic.
There may also be a subschool of binding magic used to control and coerce invoked entities … or there may not be.
Getting rid of the invoked entity is another problem altogether.
Invocation may overlap with theurgy - particularly in shamanism or in practices like Kabbalah or angelomancy. Arguably this is also how someone becomes a witch - by invoking a lower power (whether as part of an act of worship or not) and bargaining for its patronage.
It may also be a prelude to something similar to summon magic by which the user bargains for assistance at some point in the future.
Game and Story Use
- Most systems handle this sort of thing pretty unevenly - it has the capacity for some deep RP, but on the other hand you need a decent mechanistic basis in case of problems.
- Eminently subvertable when writing a magic system - especially for players used to a more user friendly system of summon magic. Knowledge of The Charles Dexter Ward Principle is extremely helpful in this sort of case.
- Also note that even if you do learn the ritual to ward off the thing you're invoking and the one to get it to go away again afterwards, you have still advertised your existence to a supernatural being that probably didn't appreciate being disturbed…
- Many aspects of how this works will depend on the nature of supernatural entities in your setting - if, like medieval theologians, you considered a great many supernatural powers imminenent to the real world and easily called across a very thin boundary then invoking them will almost certainly be easier than in the sort of RPG cosmology where the invoker must punch a hole through the dimensions.