The Extra Dungeon is a location in Devil Summoner: Soul Hackers.
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The Extra Dungeon is accessed through the "Extra Game" option on the title screen, although the player must have cleared the game first. Upon choosing the option and loading a save file, the protagonist is greeted by a voice who tells him to test his abilities. He is then dropped into the dungeon's hub area, with a Guide Room to the left and a Healing Space on the right. In the Guide Room, the voice explains the goal of the Extra Dungeon: go into each sub-dungeon and defeat its boss. Once all five of them are completed, the Exhibit Hall is unlocked.
Dungeon P
The attendant of the Theater welcomes the protagonist and tells him that there are tiles which turn into pits after being stepped on. If all of them are stepped on, the hero is teleported in front of the boss's lair, the Room of Heroes. Within it, Inui, the ghost of an imperial soldier, summons the "Inui Platoon" and attacks.
Dark Planet
The hero finds himself in an area similar to the Astro Museum, where voices can be heard, some of which are that of people who are lost in the sub-dungeon. The protagonist eventually finds a pit which leads to the Pantheon, where Pallas Athena congratulates him for navigating her "temple". She then reluctantly fights him, stating that she "doesn't battle for the sake of battle".
Quantum Leap
In Quantum Leap, the floor automatically moves the protagonist to the next room. There are also voices which teleport the teleport the hero back to the entrance should he choose to listen to them. At the House of Mirrors, Sid Davis is encountered and threatens to kill him. In the battle against him, he summons the same demons that the protagonist has. After his defeat, he swears revenge on the hero and fades away.
Sustenance and Destruction
In this area the hero is constantly floating up and down, which is needed to cross over fences that block his path. There also turntables which sometimes warps him across the dungeon. There are two different bosses to be encountered, although only one boss is needed to be defeated. In the northwestern corner, Shiva is fought, who condemns humanity's arrogance. In the southeastern most room, Vishnu appears and battles the protagonist.
Hidden Waterfall
The door to Valhalla, the lair of the bosses in this dungeon, is sealed and is only opened once the lair amongst the four sages from the Ghost Waterfall painting is discovered. Within Valhalla itself, Heimdall appears along with Thor and Tyr and fights the hero. Afterwards, Odin takes his place and, upon his defeat, says that he will fight him at Ragnarok.
Exhibit Hall
Before the five other sub-dungeons are cleared, the door to the Exhibit Hall will just throw the protagonist out. The dungeon's gimmick is that once a door is opened, all of the others open. If an open door is entered, all of the doors will shut. This is needed to go through the yellow door which leads to the last boss, "Kyouji Kuzunoha". If he is defeated, a message will appear congratulating the player for finishing the Extra Dungeon.
In the 3DS remake, two unexpected challengers appear: Raido Kuzunoha the XIV and Raidou Kuzunoha the XIV. Initially, Raido summons some of his own demons (with enhanced abilities far beyond the kind normally encountered in game). In the second phase, Raidou appears and the two engage the party directly. Upon defeat, they decide to double summon the Soulless God Oumagatsu, which serves as the Ultimate Boss.