Magicule

Magicules 「魔素, Maso」or Magical Energy, or Magic Essence, is another type of energy, or a special type of energetic material, apart from the regular Physical Energy that permeates throughout the Tensei Shitara Slime Datta Ken World. It is described to be the building blocks of all life. The term "Magicule" is derived from "Magic" and "Molecule."

Although a special kind of energy, it is still able to have causal effects on regular physical matter and physical phenomena as well. Most if not all Spiritual Lifeforms are entirely made up of Magicules and thus physical attacks do not have any proper effect on them. One would need magical attacks to cause proper damage, an example of which would be when Elen was firing Icicle Lance at Ifrit.

Magicules are analogous to Mana, Magic Point or MP in games and other shows. Magicules, however, are more accurately 'Magic Power ' rather than 'Magic Point', since even if the World is game-like, it isn't an actual game.

Although appearing to be consistent and in harmony with Physical Energy, underneath, the laws of Magicules are actually pretty broken.

Consistencies

 * If one runs out of Magicules, they die. This makes sense because if a person can no longer generate enough energy to sustain their bodily functions, they die in real life too. As such, since Magicules are described to be building blocks of life, it would mean they are akin to lifeforce energy, which if it runs out of, would cause immediate death.
 * Transforming into a being with more mass requires an equivalent amount of Magicules to be spent to make up for the mass deficit. This is most prominent when Rimuru uses his Mimicry to transform into larger creatures. The Black Fog is the excess Magic needed to create the rest of the body. In other words, the law of conservation of energy.
 * Regions with higher Magicule concentrations produce plants, minerals, and materials of higher quality, and spawn stronger monsters. Basically, the more fertile the land, the healthier the creatures.

Inconsistencies

 * Magicule recovery. Physically, people need food to recover the energy lost. Excluding potions, beings would need regions of high magical concentrations and amounts, or longer periods of time to recover high loss of Magicules. But Rimuru doesn't require that much time. Even if he took the emergency energy from Veldora, he would need to absorb enough energy to give back to Veldora for him to be in optimum condition after his release. But that isn't done and Veldora 'somehow' has all his magical energy back.
 * Mana Breeder Reactor. An Intrinsic Skill that literally 'generates' an indefinite amount of Magicules.

Inanimate Substances
Although Magicules are described as the building blocks of life, they are also capable of affecting inanimate objects as well. In most cases, only in regions with high Magicule concentrations, such as the Cave of the Seal and Rimuru's Stomach, inorganic and inanimate substances can absorb Magicules to become an organic substance, such as Demon Steel. In such cases, the substance remains an inanimate object but obtains features similar to that of organic matter, such as reshaping according to the user's will etc.

Rankings
In the World, Yuuki Kagurazaka created a system of ranking monsters according to how much Magicule or EP, they possess. Higher the EP, the more dangerous the creature is and thus the higher the ranking. '''However, it must be noted that throughout the series, individuals were assigned EP values and Ranks instantially, and it was never shown exactly how the EPs are divided amongst the Ranks. Meaning, readers are prone to guesswork.'''

Trivia

 * The unit of Magicule is EP (Energy Point).