|This article is about a/an artifact in Ninja Sentai Kakuranger.|
The Seal Door (封印の扉 Fūin no tobira) is a special door that links together the human world and the "Underground World" where the Youkai were sealed in the past.
The Seal Door has the appearance of a huge gate with two doors that open outwards; one side being the human world, and the other the realm where the Youkai were sealed. There is a special switch key near the door first seen which can only be activated by one of the five descendants of the ninja which locked the Youkai away in the past. During the final battle with Daimaou, the door is revealed to be created by the Three God Generals and, when completely closed, is ultimately sealed by the five Doron Changers, preventing the possibility of it ever opening again.
The main Seal Door was utilized during the previous battle between the five legendary ninja and Youkai when the team finally pushed their leader, Nurarihyon, into a corner. Using their power, they create the door and push him in before sealing it, defeating the Youkai threat at the time. With the door between the worlds of humanity and the Youkai shut, the Youkai remaining on the Earth's surface rapidly lost access to their Yoki and became leaderless and weakened, though not destroyed, forcing them into hiding until the day the door was reopened.
400 years after Nurarihyon's fall and the defeat of the Youkai, the Youkai Kappa discovers the descendants of two of the five legendary ninja, Sasuke and Saizou, as naïve kids who had no idea of their connection to the past defeat of their group. Using a package delivery to lure them to a certain island, he tricks them into reopening the Seal Door. Upon doing so, a swarm of Youkai re-emerge from within as well as a massive cloud of Yoki energy that re-empowered the beings. Because of this action, Sandayuu Momochi is forced to take in Sasuke and Saizou and make them and the descendants of the three other legendary ninja to become the Kakuranger in order to defeat the Youkai threat in the modern day.
At the very end of the battle with the Youkai, the Kakuranger and the Three God Generals once again return to the island and recreate this particular Seal Door, upon which they finally push Daimaou, the leader of all Youkai, inside in order to defeat him once and for all. Once sealed away completely, the ninja use their own Doron Changer as keys to keep the door shut for the rest of eternity.
Daimaou's Seal Door
A second, separate Seal Door was created and used 2000 years in the past by the three beings who ultimately became the Three God Generals in order to seal the true leader of the Youkai, Daimaou. In order to bring him back to become the Youkai's leader once again, Gashadokuro, the powerful Youkai's own son, embarks on a special ceremony to create this special Seal Door for bringing his father back from his confinement.
In order to create this Seal Door, he uses Umibozu, a powerful Youkai, to specifically seal children at certain points and use their spiritual energy to create the door; using their energy, the door is created with Daimaou needing a full month to break through and emerge from within after it appears. When the Kakuranger attempt to intervene, Gashadokuro further protects the door point by sealing all nearby humans and raising the special Seal Door above the sky to prevent them from reaching. Due to the weaker position of the ninja heroes; the Kakuranger must seek out the scrolls containing the Super God Beasts in order to further their own training and stop the ceremony. Once used, the Kakuranger ultimately defeat Junior and save the civilians, but are too late from preventing Daimaou from emerging through the door and becoming the Youkai's leader once again.