Rite of the Void

The Door of Demons

The Rite of the Void is a ritual ceremony that is used to summon a Demon. Generally, it cannot be used to summon a Demon Lord unless performed to the highest level, perfectly. Instead, it summons minor Demons.

The Ritual

The start of the ritual is, of course, the ritualistic sigil that one must draw with their own blood.
Each sigil is different, and there are specific minor differences that can put one into contact with a different Demon. The wrong line, the wrong symbol, and one is not doing exactly what they may think they are doing.
After that, study is required. To connect in some way to the Void, most commonly through attempting to read the Demonic Texts, a series of untranslatable works in an indecipherable language.
It was breaking through these texts in the smallest of ways that even made this practice more accessible, though the one who did so regrets ever making it public knowledge due to the cost.
Only one major line from the Demonic Texts has ever been translated, as far as the wider world is aware. Count Weird wrote a translated line that read:

“To contact a Demon, you must open the door between the plane of the living and the Void... a door known as the heart.”

— Count Weird

Count Weird by RovaRed

Sacrifice is the key. Sacrificing something that is dear to the summoner is said to open the door to the Void.
That sacrifice crumbles into nothingness, being taken by the Void. Their soul, if they are alive, shall live in everlasting nothingness within the depths of the Void in return for the power the summoner gains.

Minor Demons

It is almost always the case that this intense ritual will not grant eternal power, but instead bond a Minor Demon to the summoner through Demonic Possession.
The most well known case of this was the Mamman known as Count Weird, who went through intense study and sacrifice to summon the rubber Demon, Gaorast.
While some have been disappointed in this, a Minor Demon is much more manageable than a Demon Lord. Demon Lords are powerful and take control, cursing their users.
Minor Demons generally do not have the power to do that, and instead grant power for small amounts of control over the user and a mutual agreement between them.

Demon Lords

The Cult of the Void, an offshoot of the Faith of the Unfaithful, have been said to have performed the Rite of the Void to summon a Demon Lord on some rare occasions.

Regidagonus by Yumedatchi

How exactly they pulled it off is unclear. Some think that by growing close to each other and willingly sacrificing large swaths of the cult, they are able to put more power into the Rite, thus reaching further into the Void until a Demon Lord can hear them and is willing to follow their call to the surface.
There is a legend, of the Elven Captain Aeydark the Hunter, that he accidentally performed the Rite of the Void during the Ilder Massacre, thus summoning Regidagonus to the mortal plane through his body.
Aeydark was a hunter, and invaded Ilder, cutting apart the residents and sustaining wounds of his own. It is believed that enough of his blood spilt around the village to create a sigil for Regidagonus, and that he had killed enough people, of whom he considered worthy prey, that the sacrifice reached the Demon Lord. Due to Aeydark considering the hunt the most important thing to himself, the prey were worthy sacrifices.
Thus, the destroyed Ilder was the site of a ritual.
However, this is not the only way that a Demon Lord can be summoned. Demon Lords can be brought into the body through many means, such as alligning perfectly with what they seek, or simply pushing oneself to ones limits until a Demon Lord can break through the seal within the soul.
Always, however, they enter the same way. The Door to the Void: the Heart.