Disgaea 2: Cursed Memories/Dark World

The Dark World could be best described as an alternate dimension to the main story stages. All main stages (excluding the final boss fight) have a more difficult "Dark World" version, it's added challenge often thanks to the Dark Sun. By completing certain requirements on each stage, the Dark World equivalent can be opened. However, you must complete the stage normally first before being able to unlock the Dark World version, and you must unlock the Dark World dimensional guide before anything.

Opening the Dark World
Five hidden switches are scattered throughout Holt Village, each activated by pressing psx: X while Adell is in their general location. When one is pressed, a sound effect will roll and a dialogue box will pop up. All must be activated to awaken the Dark World dimensional guide, Friday MKII. The locations are listed below, and don't worry, they can be pressed in any order. Some can't easily be described with much accuracy, but you' should find it fairly soon if you just move around in the general area and repeatedly press psx: X.
 * Behind the Red Skull (character who awards you Subpoenas), at the corner between the broken pillar and the edge of the map.
 * Near the Ghost character, somewhere between the trees and the map's corner.
 * Behind the house near the Zombie character.
 * In the area of the Wood Golem and Mothman, there should be a break in the brick railing. Jump over this so you land on the small grassy area on the opposite side.  Face the break in the wall, and you'll be standing right above this switch.
 * Behind Adell's house, in the concave corner where two walls intersect.

If you can't find one of the switches, don't worry, just move onto the next one. You could have forgotten you pressed it previously. The activated switches won't reset until you start a new cycle.

Once all five are pressed, a dialogue box will read something along the lines of "A dark shadow looms over Holt Village". Friday MKII, a shadow-version of the Archer class, will suddenly appear near the Ghost character. Initially, all she will say is "...". Run off to complete any type of battle, and then return to speak with her again. This time she will ask you a question, answer yes to fight her right there in Holt Village (or answer no, if you don't think you're strong enough yet). Her defeat will allow you to use her as the Dark World dimensional guide, but let's not get ahead of schedule, first you'll have to fight her.

Friday MKII's equipment and stats will vary slightly whenever you face her, but she is always Lv100. If this is your first run through the game with this save file, defeating her could be a challenge (but the Dark World's difficulty might be too much if you can't win, so there wouldn't be much point anyway). If you're having trouble, remember that as an Archer, Friday MKII's RES is generally higher than her DEF, so it would be best to use ATK and HIT-based attacks instead of spells. If you have difficulty reaching her with several units because of the distance between your base panel and Friday MKII, take out a defensive character (such as Heavy Knight or Fighter, either equipeed with multiple armors) to lure her out into proximal range. In any case, remember to utilize combo attacks, and keep your characters separated to avoid multiple units becoming victim to her Zielregen area-of-effect attack.

If you're only around Lv20 or so and don't think you have a chance to win, take out a Thief (or several, if you have more than one) with equipment focused around HIT. Make sure you can use the class-specific specials Steal Consciousness, to put her to sleep, and Steal Health, to slowly drain her HP with poison. You should first focus on putting her out of commission with Consciousness, and then slowly drain away HP with Health. Steal Freedom isn't much of a help, because she's still likely to attack due to the long range of her bow. Inflicting Amnesia with Steal Memory could be beneficial to take away her stronger attacks if she wakes up, but it should take precedence behind Sleep and Poison. If you still have trouble hitting her with the Thief's specials, use a long string of combos, with Steal Consciousness last in the line (don't use this method while Friday MKII is asleep, as the damage could wake her up), to significantly boost accuracy. It doesn't really matter if the other attacks used do damage or even hit, as they'll still add onto the combo's accuracy bonus.

After her defeat, Friday MKII will appear as the Dark World Dimensional Guide next to the regular Dimensional Guide, Friday. Be warned that she only appears in Holt Village, not the maps found in Chapter 6 or Chapter 7. Now you can speak with her to access the Dark World stage selection, but Stage 1-1 will be the only available stage for now.

How to Unlock additional Dark World Stages
As stated earlier, to get more stages available in the Dark World selection, certain requirements must be passed for each individual stage. You must first defeat the stage normally, and then enter it again to pass the Dark World unlocking requirements. Upon completing some requirements, you will be instantly transported to the Dark World version of the stage. On others, you will be prompted as to whether or not you want to go the Dark World stage you just unlocked - If you select no, the stage will still appear in Friday MKII's selection. Of course, it will also appear if you defeat the stage, but not if you are defeated. "Optional" transport stages usually require you to defeat all enemies in a certain, while "Instant" stages typically only need you to perform a specified action. All of these stages will disappear from Friday MKII's selection after a new cycle begins, and you will have to unlock each individual one again if you wish to clear them another time.

Dark Sun
The infamous Dark Sun is an aspect unique to all Dark World stages, a giant flaming orb hovering above every map.

Every turn, excluding the first, the Sun will activate a special action to weaken your group. These actions can range from inflicting status ailments to all of your female characters, inflicting direct damage to your units, to even activating an instantaneous Game Over. What effects take place depends on the stage and current round, but you can check and plan ahead for the "Dark Sun Report" by holding in psx: S anywhere on the map when you're not highlighting a character.

If enemies have numbers above their heads, that means they intend on sacrificing themselves and ascending up to the Dark Sun in the specified amount of turns. A "1" signifies they will ascend next turn. Upon ascending, that foe's level will be added onto all enemy characters currently on the map. If you defeat foes before their timer reaches zero, they will not ascend, and your enemies will not level up because of it.

Your foes aren't the only ones who can ascend, as a unique "Ascend" option appears in the action menu while in a Dark World stage. By selecting this, the chosen character will sacrifice themselves and ascend up to the Dark Sun, and attempt to infiltrate and destroy it. It's uncertain what determines how great the Ascendant's chances of winning are, but warrior-type characters seem to have the best chances, and wizard-types the worst chances. Levels and stats also seem to have an effect; the stronger your stats, the greater your Dark Sun-destroying ability. If you create a tower and select "Ascend" with the bottommost character, only the top character will be sent off, but with a significantly boosted chance of destroying the Dark Sun. Even if one of your characters fails in their destruction attempt, they will take down the Dark Sun's strength (as noted in the "Sun" sidebar, under the Bonus Gauge), and make it easier for another to Ascend and destroy the nuisance. The "difficulty" stated at the beginning of each stage directly correlates to how much damage the Dark Sun can take.

Ally characters with Felnoies who ascend to the Dark Sun won't do any damage. Instead, their level will be added to all enemies' levels, like when an enemy ascends.