The Six Ancestors' Great Seal is an ancient technique which was used in the aftermath of the Great Monster Wars to seal Alice I (First) and the Six Ancestors, and is the most powerful seal in existence.
The seal draws on the power of its target to sustain itself, meaning the more powerful the target, the more effective the seal. After being placed on a target, the seal weakens their power and can subsequently teleport them into an area of subspace completely locked away from the outside world, rendering them useless.
However, there is a loophole, as discovered by Tamamo: if the target weakens themselves sufficiently, they can escape the seal. Since the seal feeds off the strength of the target, if the target's power decreases, then so does the power of the seal, as it was only intended for use on incredibly powerful beings. After weakening their power, the target may create a vessel of themselves in the body of a child which they can use to escape the dimension while leaving their original body trapped inside the subspace barrier. While the target retains their mind and memories, their weakened state renders them at less than one tenth of their original power. To temporarily remedy this loss of power, Tamamo developed Word of Dispel, which would revert her body and power back to its original form for a few seconds.
There is only one way of truly escaping the seal, and that's by destroying the source of energy. But since the source of energy comes from the one being sealed, the target would have to kill themselves to weaken its power, which is hardly ideal. In Alice XVI (Sixteenth)'s case, the seal was cast by a device and not by Alice herself, as Promestein could only replicate the technique through technology. As such, the seal could be broken once the instrument became vulnerable. However, Alice I and the Six Ancestors are mentioned to have added a counterbalance to their seal: they agreed to seal themselves under the promise that the monster race would be unharmed, a mass genocide committed against them would quickly saturate the atmosphere from their released dark magic, and cause the seal to break.
After Goddess Ilias had threatened the very survival of the world and the creatures she and Alice I created due to their constant clashing in the Great Monster Wars, she proposed that if Alice I and the Six Ancestors were to seal themselves, she would not harm the monsters. The ancestors were forced to comply and applied the Six Ancestors' Great Seal on themselves, however, if Ilias were to break her promise and began to exterminate monsters, the heavy saturation from the dark magic released from the deceased monsters would cause the seal to break. However, seemingly unbeknownst to Ilias, Alice I secretly ordered Tamamo to escape the seal to monitor the Monster Lords from the shadows, using a weakened form to hide from Ilias.
However, Ilias had no intention of keeping her promise and spent many years of coming up with a way to exterminate monsters without removing the seal on Alice I. To this end, she recruited Black Alice, a descendant of Alice I with an intense desire to rule the world, and Promestein, the creator of the artificial monster race chimera, to keep the darkness in check.
A thousand years following the Great Monster Wars, Promestein successfully created a device that replicates the Six Ancestors' Great Seal, using Queen Ant as a test subject for the seal and left Grangold to monitor the seal. That version of the seal proved incomplete, as Queen Ant eventually broke free. It is mentioned by one of Grangold's magic scientists that Promestein intended to create a seal to use against the Monster Lords. Later, after Luka rebels against Ilias and the latter begins to remake the world, Promestein applies a completed version of the seal on Alice XVI (Sixteenth), the current Monster Lord. However, she later manages to escape the seal with Tamamo's advice, but Promestein boasts that even she cannot remove it and taunts her by saying Alice would have to die to lift the seal.
Tamamo searches through Promestein's records from her secret laboratory under Remina, but none of them holds any information about Alice's seal. Instead, her aide Nanabi detects a faint presence of the seal's wavelength in the Forest of Spirits. The forest is revealed to secretly house the Biolabs, where Promestein's aide La Croix guards a device powering Alice's seal. However, the device itself is fortified with extremely pure holy energy, meaning it cannot be physically touched, not even by the Angel Halo. Fortunately, Tamamo later comes up with a solution to distort the holy energies throughout the world using the Four Towers, allowing holy entities including the sealing device to be harmed. The plan is successful, and Alice manages to regain her original body.
As the war reached its climax, Promestein, along with Black Alice, betray Ilias and reveals that the true purpose of the Great Seal's recreation was to use it against her all along, knowing full well she is vulnerable to its effects just like Alice I. This gives an opening for Black Alice to use the White Rabbit on herself to absorb Ilias, becoming the ultimate entity of light and darkness. However, once Luka and Alice defeat Promestein and Black Alice, Ilias emerges from the latter's remains, revealing that her true master plan all along was to become the ultimate entity herself from being engulfed by Black Alice, giving her the power to defeat Alice I if her seal was to be removed. Despite her new power, Ilias too falls, preventing the final stages of her plan from coming to fruition.
Not long before the beginning of Luka's adventure, the Monster Lord's castle has been attacked by the White Rabbit who proceeded to use the seal on Alice, forcing her to step down from the throne to leave the place free for her mother without knowing it. The sealed Monster Lord, enraged by her predicament, starts to chase the White Rabbit with or without the support of Luka.
In the meantime, a small version of Ilias appears and crashes near the top of Iliasville Mountains. After failing to smite Luka, she realizes her power has been weakened, and thinks that someone else is responsible for casting the Great Seal upon her. In reality, she's a small part of the Goddess Ilias of the original world, and ironically survived because a small part of herself managed to escape the seal and her own destruction afterwards.