Extra Stages refer to special battles that can be undertaken by meeting certain requirements and passing respective bills in the Dark Assembly. After being unlocked, these stages will appear as selectable areas in the Dimensional Gate menu, and will disappear if cleared or if you begin a new storyline cycle. Some of these stages provide secret and unique items or characters after clearing them, but most are just references to other Nippon Ichi-developed games.
Return of the Prism Rangers
In Chapters 12 and 13 only, you can open this stage by approving the "Return of the Prism Rangers" bill in the Dark Assembly for 200 mana. As you may have expected, the Prism Rangers will return for one final battle against Adell, but their team now has all seven colors. By this stage of the game, you'll have no problem defeating them, and there is no reward outside of a normal battle aside from receiving the "Prism Black" title for Adell. If underpowered, be sure not to leave Prism Indigo as the last enemy to fall. His statistics double when he is the last enemy on the map.
In one cycle, use up three Defender of Earth cell phones to reveal the "Rescue the Earth Defender!" proposal in the Dark Assembly. As a Senator, have Adell pass this bill to open up an extra stage.
Rescue Kurtis Again
In the same cycle, after clearing the "Rescue Kurtis" stage, use up another three Defender of Earth cell phones to reveal this proposal. As a Senator, have Adell pass this bill to unlock another extra stage.
In the PSP Port, Dark Hero Days, completing this mission unlocks the Kurtis duplicates (Copy Robots) as a playable class.
With at least 20 hours of gameplay in Chapters 1-9, the bill "I want to see the ending!" appears. Pass this for 400 mana to unlock a fight against Vyers or "Mid-boss", better from several appearances in Disgaea: Hour of Darkness. He's around level 60 or 80 and possesses a few weaker allies, and isn't much of a challenge if you've defeated the game at least once. His defeat warrants a unique and quick ending to the current cycle, but it contains little more than the average credit scroll.
With 33 felonies, the bill "I want to fight a Hero from Another Game!" appears. Pass this for 100 mana to fight Asagi, a character who later appears in the game Soul Nomad. This can only be passed while on at least the third cycle.
With 66 felonies, Chapters 1-9, the bill "Unleash the Fallen Maiden" will appear. Pass it for 100 mana to fight Prier, the heroine from La Pucelle Tactics.
With 99 felonies, the bill "Break the Mysterious Seal" appears. Pass this with 1500 mana to fight Marjoly, the witch antagonist from Rhapsody: A Musical Adventure.
In Chapters 12 or 13, with at least 40 hours of playtime, the bill "I want to fight an Overlord!" appears. In the stage, Laharl and Flonne from the original Disgaea will fight you, and they'll have the assistance of a tier 6 Zombie and tier 6 Dragon. Flonne and the monsters will be Lv500, and Laharl himself will be Lv600. It would be best to defeat Flonne first, as she can heal the others. After this stage is cleared, Flonne will ditch Laharl and join your group until the end of the current cycle.
Summon Laharl Again
After Flonne joins you in the Overlord's Decent stage, the proposal "Summon Laharl" will appear. With Adell as a Senator, pass this bill to unlock another stage. This time Laharl is Lv1000, and is assisted by six Lv800 humanoids. After clearing this stage, Laharl will join you until the end of the current cycle.
In Chapter 1 only, pass the "Pick a fight with an Overlord" bill for 5000 mana to fight Laharl again. A new cycle/ending will be attained once you win this battle.
With 10 dark world maps cleared, the "Meet the Strongest Overlord" bill will appear. Pass this bill with 5000 mana to unlock a fight against Lord Zetta, the protagonist from Makai Kingdom. Adell will receive the "Badass Overlord" title from Zetta upon winning the battle.
Fight Baal, Prinny Baal, and Uber Prinny Baal
With 25 dark world maps cleared, the bill "Meet Baal the Tyrant" will appear. Pass this bill with 9999 mana to open a stage to fight Tyrant Baal, at Lv4000. For defeating him, you will earn "Makai Wars", one of the best equipable accessories available.
Unlike other extra stages, you can return to this stage after clearing it. If you do so, Tyrant Baal will be replaced with a Lv8000 Prinny Baal with around 23 million HP, and he will now be accompanied by four Lv4000 Tyrant Baals. Unlike most extra stages, this can be cleared multiple times.
Uber Prinny Baal is the Land of Carnage version of Prinny Baal, and is approximately twice as powerful as Prinny Baal. He is still accompanied by Four Tyrant Baals, all of which have also received a significant boost in strength. This stage can be cleared multiple times. After defeating him once in the Land of Carnage, you may encounter him as a Pirate while in the Land of Carnage's Item World. If you can defeat him there, he may reappear with four Baal clones. Defeat him again, and Adell will earn the "Lord Master" title.