How does your fictional world’s summoning or conjuring work? When your magic users call for something to appear, what’s going on that makes it work?

Here are a few sample models I’ve seen in fiction. Each approach implies different things about how to break the summoning, and the entity’s attitude toward being summoned. They can also be combined.

• Appliance. Summoned entities need to be gifted enough energy by the summoner to form a body with enough capacity to contain their personal power. Anything that drains this energy, or interrupts its accumulation, can disrupt the summoning. Whether the entity wants to be summoned or not, the energy is required. The energy itself might be raw magic, the presence of elemental energy, the power of blood sacrifice, etc.
• Vessel. A being, effigy, or item must be prepared as a container, magnet, or anchor for the entity. This could be the last daughter of the ancient bloodline of queens. It could be a special gem the summoner has prepared. Anything that damages or disqualifies the vessel can break the summoning. The summoned entity might desire to enter the world through this vessel, or might be unwillingly trapped so long as it exists.
• Unlisted Number. The entity is either willing or compelled to appear when some rare bit of knowledge is invoked. This could involve a “true name”, a special prayer, the invocation of an oath which binds the entity, and so on. It’s difficult to break a summoning like this unless you interfere with the summoner themselves, or somehow remove their memory of the secret. The invocation itself may contain escape clauses, such as counter-invocations, or rules the summoner must obey to retain the entity’s presence. An entity that doesn’t want to be summoned will take steps to remove this knowledge from the world (such as trying to kill any summoner!), while those that want to be summoned might still want to be choosy about who responds, and as they say, knowledge wants to be free.
• Stars Aligned. An activity or ritual at the right place and/or time will attract the summoned entity. This includes summons which only work at midnight, or on certain holidays, in consecrated areas, and so on. Generally the summoner has no control over the place or time - be there, or be square - and so interfering with the summoning means controlling where the summoner can go. Once the entity is here, all bets are off unless there’s an escape clause. People opposed to the entity may set guards around such places, or during such times.
• Pact. The ability to summon the entity depends on the summoner’s behavior. This can be because the behavior itself is significant (Elgabar the Unsullied can’t appear before anyone who hasn’t stayed ritually pure for seven days), or because the entity is somehow able to monitor the summoner’s actions. It’s generally hard to stop summons like this from appearing, unless you can make the summoner mess up.

Generally, the more powerful the entity, the stricter the requirements for summoning. For example, it might be possible to beckon elemental spirits anywhere there’s a strong source of the element (Appliance), but to conjure the Ocean Lords requires both a special place (Stars Aligned) and a ritual naming them (Unlisted Number).

What happens if a summoning goes wrong?

• All or Nothing. If you don’t satisfy the requirements, the entity can’t or doesn’t appear.
• Corruption. The entity might appear, but in a mutated or damaged form. It might turn on its summoner, go feral, or otherwise act in undesirable or surprising ways. If the heroes thwart an antagonist’s summoning, they may still have to contend with a corrupted outcome. An example of this appears in 1984’s “Conan the Destroyer”.
• Diminishment. The entity appears, but in some smaller, weaker, or lesser form - a grand lion appearing as a tiny cub, for example. This is a common comedy outcome for protagonist summoners, especially in anime.

What are some variations on the basic idea of summoning?

• Asymmetric Summons. This is where two or more forces in opposition have different rules for summoning their chosen entities. For example, imagine two warring nations, Deblan and Astar. The Deblanites can summon their ancestral champions at sacred places found around the landscape of both nations, while the Astari can conjure the Night’s Children anywhere they can accumulate enough magical energy, but only on certain holy days of the calendar. The Astari will wish to deny the Deblanites access to their sacred sites, while the Deblanites will build up forces in preparation for the predictable but perilous onslaughts on certain days.
• Contested Summons. What happens if two people try to summon the same entity at the same time? What happens if they’re on opposite sides? You can say whoever got there first wins, or whoever puts more oomph into the ritual wins. It’s also possible that two summoners will try seizing control from each other, resulting in an entity that doesn’t get much done, but might cause a lot of chaos in the process.