Harvest Festival

The Harvest Festival (魔王ヘの進化) is a phenomena that occurs once all the requirements to become a True Demon Lord have been met.

Harvest Festival Requirements
In order to meet the base requirements to undergo a Harvest Festival and become a Demon Lord, an individual has to fulfill certain conditions, at which point they're referred to as a Demon Lord Seed (魔王種). Once the individual obtains the qualifications to become a Demon Lord, they're required to Germinate the Seed (必要な条件) using a large amount of Souls.

It is usually said that 10,000 Human Souls are necessary for this process, but in fact, the soul of any creature capable of passing a mirror self-recognition test is sufficient. Similarly, the 10,000 number is only an average, with the actual quantity of soul energy being what matters, not the amount of individual souls.

Effects
Once an individual with the qualifications to become a Demon Lord germinates the seed with an appropriate amount of Nutrients (人間の魂), the World Language will confirm their intent to become a Demon Lord, after which, the start of the Harvest Festival will be announced to those in their surroundings and the Initiation (通過儀礼) will begin.

The Initiation is a process in which the Demon Lord Candidate is tested by the world itself to determine if they're worthy of being a Demon Lord. The individual's consciousness will grind to screeching halt as they're put into a special "sleep" that is impossible to resist and lasts for three days. This "sleep" is different from the usual Evolutionary Slumber and from the Low-Activity Mode used by Spiritual Lifeforms. During the duration of the "sleep", the individual will be left completely helpless, with even their defensive skills granting them no benefit. If one is not careful, they could be assassinated by an enemy or disloyal ally while undergoing the initiation, making undertaking it a massive risk.

During the duration of the Initiation, the individual will start the main course of the Harvest Festival in full. Their body will be reconstructed in to a stronger state, usually resulting in the individual's Super Evolution (超進化) into their desired form and acquiring powerful Intrinsic Skills along with it. The individuals mind will also be stimulated, making the individual a Spiritual Lifeform in mind if they weren't so already. This can potentially result in minor personality changes, although the odds of that are slim.

The power granted to an individual upon becoming a Demon Lord is immense, so much so that it isn't uncommon for True Demon Lords to possess Divinity, making them super-high level existences with increased odds of awakening an Ultimate Skill in their near-future. It is also possible for the individual's Equipment to increase in quality by leaps and bounds, potentially raising it to God-Grade or near God-Grade.

Gifts
Upon the completion of an individual's Harvest Festival, Gifts (祝福) will be granted to everyone in that individuals Soul Genealogy, most notably those that they've named. The Harvest Festival's "Gifts" can come in many forms. Some gain Skills that they desire, potentially even awakening Unique Skills, while others attain evolutions that would otherwise be impossible, such as a Hobgoblin becoming an Ogre. Whatever the result, it is almost certain that their "wish" will come true.

Artificial Harvest Festival
During the events of The Imperial Invasion, Raphael devises of a method of artificially awakening individuals who would normally possess the qualifications to become Demon Lords, but are unable to become one due to being subordinate to someone else, utilizing a ritual that mimics the effects of the Harvest Festival. The process requires ten times the normal amount of soul energy, using Beelzebuth to break it down into pure energy and then giving it to the individual in question. As a result, a Harvest Festival-like phenomena is created, with the target individual undergoing an Awakening Evolution, making them an existence equivalent to Demon Lords.

This artificial evolution is complete with Gifts, although the range of people that can receive them seems to be more limited than the genuine Harvest Festival's. Meanwhile, it also comes with a few other advantages, such as the "Initiation" being replaced with regular Evolutionary Sleep, which can be resisted with relative ease.