This is a list of Unlockable Stages in the Disgaea series. Most can be unlocked through the Dark Assembly or that game's equivalent.
Disgaea: Hour of Darkness
Alternate Netherworld
Unlocked by passing appropiate bill in the Dark Assembly. Legendary Overlord Priere can be fought in the deepest parts of this dungeon and will subsequently join the party
Prinny Land
Used to unlock the different tiers of Prinnies. Unlocked by passing the appropriate bill in the Dark Assembly after Chapter 8:Reincarnation.
Cave of Ordeal
Unlocked by passing the bill in the Dark Assembly. Once cleared it can be revisited (in the PSP and DS versions) to play through the Demonhall Mirror, where Adell and Rozalin can eventually be fought as super-bosses. They join the party in Disgaea DS after defeating Rozalin after Laharl complains that he will always be the main character.
Human World
Unlocked by passing the corresponding bill. Unlocks the Human World Conqueror ending if cleared.
Beauty Castle
Unlocked with the Mysterious Seal bill. Marjoly the Most Malevolent Witch in the Universe can be fought here. She joins the party afterwards.
Baal Castle
The ultimate super boss Baal can be fought here. It is unlocked after beating the other unlockable stages. After beating him once, he can be fought in Prinny form, which is much more difficult to defeat.
Disgaea 2: Cursed Memories
Dark Sun World
Trigger 5 hidden switches within Holt Village with the X button. A new NPC named Friday MK II will appear. Defeat her and she will now become a dimensional gate for the Dark Sun World. Dark Sun World maps are unlocked by performing certain achievements in storyline maps.
Land of Carnage
Defeat 16 different pirate groups for treasure maps withing item worlds (some of them only found in item worlds of legendary items). You may also steal those maps from them. After obtaining all maps, a brief cutscene will happen in Holt Village. Talk to Hawkeye to unlock the new ship.
Cave of Ordeals
Unlocked after gaining 700 mana, and passing a bill at the Dark Assembly.
Summer Greetings
Get 33 Felonies on a Single Character and pass the bill "Fight a Main Character from another game". Allows you to battle Asagi.
Ruins of Saint Paprika
Get 66 Felonies on a Single Character and pass the bill "Unleash the Fallen Maiden". Allows you to battle Priere. Only access able in chapters 1-9.
Beauty Castle
Get 99 Felonies on a Single Character and pass the bill "Break the Mysterious Seal". Allows you to battle Marjoly.
Prism Ranger Show
In chapters 12 and 13, pass the bill "Return of the Prism Rangers". Battle the recently completed Prism Rangers.
Darkly Adonic
After getting 20 hours of gameplay time in, pass the bill "I want to see the ending". Allows you to battle Mid-Boss. Ends the game once completed and forces a New Game+. Only available in Chapters 1-9.
Defend a Defender!
Use 3 Defender of Earth Cell Phones and pass the bill "Rescue the Defender". Allows you to battle the Kurtis Clone. Adell must be a Senator in order to unlock this stage.
Robots Attack!
Use 3 more Defender of Earth Cell Phones once "Defend a Defender" is completed and pass the bill "Rescue Defender Again!". Allows you to battle the army of Kurtis Clones.
In Holt Village
In chapters 12 and 13 pass the bill "I want to fight an Overlord!". Allows you to battle Laharl and Flonne. Flonne joins you once completed.
In Holt Village (2)
Pass Flonne's bill "Summon Laharl!" while Adell is a Senator. Allows you to battle Laharl once more. Laharl is unlocked once completed.
In Holt Village (3)
Pass the bill "Pick a fight with an Overlord!". Allows you to battle Laharl. Ends the game once completed and forces a New Game+. Only available in Chapter 1
Makai Kingdom
Clear 10 Dark world Maps and pass the bill "Fight the Strongest Overlord. Allows you to battle Overlord Zetta
Baal Castle
Pass a bill in the Dark Assembly after clearing 25 different Dark Sun World maps. Allows you to battle Tyrant Overlord Baal.
Hardcore Instructor Champloo
Pass a bill in the Dark Assembly after getting 50 hours of Gameplay Time. Allows you to battle Mr. Champloo in the Evil Academy Home Economics Kitchen. He joins you once completed. Only available in Disgaea 2: Dark Hero Days.
Hardcore Instructor Raspberyl
Pass a bill in the Dark Assembly after beating Champloo and beating 30 Dark World Stages. Allows you to battle Raspberyl in an Evil Academy Classroom. She joins you once completed. Only available in Disgaea 2: Dark Hero Days.
Hardcore Instructor Mao
Pass a bill in the Dark Assembly after beating both Champloo and Raspberyl and unlocking the Land of Carnage. Allows you to battle Mao in his room at Evil Academy. He joins you once completed. Only available in Disgaea 2: Dark Hero Days.
Disgaea 3: Absence of Justice
Land of Carnage
Pass Tiger Cave club bill with at least one of your characters in the club. This bill becomes available when you gain your first Illegal ticket (from mystery room in the Item World or by defeating pirates in Item World). A new NPC will appear in the Freshmen Building. Clear 25 different stages to unlock the Battleship Gargantua which can take you to the Land of Carnage.
Class World
Find the prinny that has an Afro. After clearing 3-3 and creating the Student Council Club in Homeroom, you will receive messages after stages cleared with Student Council Club members included in the party (repeated stages are OK for this) that Afro Prinny has appeared. He appears in random spots.
You can then go into a character's world. The level of enemies will match the level of your character. You can only go into 10 stages at a time. The bonus list of the Class World only has random XP and money bonuses, so it is a good idea to finish each stage whenever possible. These maps are rather small and have a lot of enemies. The reward of finishing Class World stages is extra base stats that will level up a random stat by 1% for each Dropout you defeat and if you beat the boss on the 10th stage. The number of times you level up roughly depends on the current teacher in Homeroom.
Reverse Pirating
Defeat the pirates of an item in that item world, and have at least one member in the Reverse Pirating club. As long you are in Floor 20 or deeper in that item world, and can only do it once per 20 levels. You can capture or remove innocents for that item.
House of Ordeals
Clear Episode 7-4 and a bill will become available in the Assembly. Pass that bill to unlock 4 new maps, one of which is a nice power leveling map. Alternatively, you may gain 7,000 mana on one character. The option to unlock the House of Ordeals will automatically open for a character with over 7,000 mana. This option will disappear if the character no longer has 7,000 mana.
Extra Stage 1
Battle Master Big Star in the Sophomore Building. He rejoins your party once defeated.
Extra Stage 2
Battle Salvatore in the Senior Building. She rejoins your party once defeated.
Extra Stage 3
Battle Prism Red in the Frozen Academy Building. He joins your party once defeated.
Extra Stage 4
Battle Asagi in the Heart Bank. She joins your party once defeated.
Extra Stage 5
Battle Marona in the Abandoned Academy Building. She joins your party once defeated.
Extra Stage 6
Battle Axel Dark Hero in the Evil Academy Theater. He joins your party once defeated.
Extra Stage 7
Battle the original Disgaea Trio (Laharl, Etna and Flonne) in Mao's Home. They join your party once defeated.
Extra Stage 8
Battle Baal in the Inter-dimensional Walkway. Harder battles with him are unlocked once defeated.
Diez Gentlemen Battles
Battle the remaining Diez Gentlemen. Each member has their own bill.