Summoning Magic「召喚魔法, Shoukan Mahou」is a branch of Magic that involves summoning a living being from another plane of reality. The most commonly used Summoning Magic is that of summoning Demons, and less often, Spirits. The most rarely used Summoning Magic would be Summoning Otherworlders simply because of the sheer time and effort it takes to prepare for the summoning. Almost all Summoning Magic requires a relatively high amount of Magicules to execute, and always involve some kind of contract or deal with the summoned individual.
Although Demons are the embodiment of evil, they are surprisingly useful when it comes to executing a plan, since they are known to value a properly approved contract. Thanks to the summoning contract demons cannot betray the summoner, given it is reasonable. However, any request beyond the original contract automatically annuls it. This is because if such an act is done, it testifies that the parties involved are not satisfied with each other.
When summoning demons, the summoner is required to provide some kind of sacrifice. This can be in the form of pure Magicules or living sacrifice. Since Demons are constantly attempting to become stronger, either can contribute to their future growth. However, in most cases, the compensations presented also used as a means to construct a physical body, since as spiritual beings, they might disperse and disappear if they assimilate without one in the physical world.
Naturally, the higher the rank the Demon is, the higher the compensation and magic need to be to summon and create a contract with them. Even summoning a Lesser Demon can utterly drain an A-Rank Adventurer, such as the instructor who tests Rimuru for promotion. If one offers too little energy in the process, the Demon might try to kill the summoner, although, that kind of thing can be fixed with a proper contract by providing it with adequate magic energy.
Higher Ranking Demons are also harder to control and bind under a contract. One must be adequately prepared since higher order demons, such as Arch Demons, who are known for their high pride and unwillingness to follow a master, or else they might ask for ridiculous reparations or such compensations that might ruin the whole point of establishing the deal, such as the unnamed Testarossa, who killed her summoner's country as compensation for killing an enemy country.
There are two known methods for Summoning Spirits: 1) is to call out to them from the altar inside Ramiris' Labyrinth. If a Spirit shows a liking or has a high affinity towards the individual, it will answer the call and appear to help. 2) Rimuru is able to gather the castoff essence of a spirit and combine them into an artificial spirit using his Skills. The spirit is then capable of being transferred to a host.
Summoned Spirits are usually harmless and have no sentience, working only by means of an abstract kind of will or directive. However, it is also possible for sentient Spirits, like Ifrit to exist as well. Although, such spirits are very high ranking and, similar to Demons, unlikely willing to serve and help just about anyone.
Spirits are also somewhat mischievous, in a childish sense. High-level spirits are said to appear before those who possibly have the destiny to become a Hero.
It was stated by Veldora that it took 30 mages, 30 days to summon an Otherworlder. And then a contract is usually etched onto the soul of the summoned to control them. It takes an enormous amount of magicules and could drain and kill the summoner if they are ill-prepared.