A lot of people familiar to the Disgaea series will say that the main story is nothing but a tutorial, as a lot of gameplay will be left to the postgame content. This page will detail how to unlock the Rakshasa Dimension, as well as what it is.
How to Unlock the Rakshasa Dimension
Rakshasa is the third difficulty that can be unlocked, after the Carnage one. In order to unlock it, you will first need to have unlocked the Carnage Dimension. Go through the Carnage Dimension area that unlocks and complete those stages. Once you do that, you should see a new bill appear in the Dark Assembly, titled Challenge the Rakshasa Dimension!. This bill will be difficult to pass normally, so you will likely need to force it to pass. If your characters are having trouble passing it, then you might want to level up some before attempting again.
What is the Rakshasa Dimension
Similar to the Carnage difficulty, Rakshasa will be a step up and it will immediately show, should you try to tackle any of the maps in the Rakshasa Dimension area. Also, all of the normal maps will now have an added difficulty you can choose on the selection screen, pressing L/R (Switch) or L1/R1 (Playstation) to cycle through them. Thankfully, you will gain yet more options during Super Reincarnation, allowing you to unlock the Lv Limit Break 2 and Stat Limit Break 2 Karma options. The former will let you level to 99,999,999, while the latter will increase the base stat max from 500 to 2,000.
To get an idea of the devastation you’re going to face whenever you attempt the Rakshasa Dimension area, the first map will have enemies at level 12,500,000, while the final map’s enemies will be at the max level. As with the Carnage enemies, the foes you face on the Rakshasa difficulty will have an Evility to increase their overall damage, compared to the other difficulties. You will need to move slowly here, as you want to unlock those Karma options listed above, then start working on getting your main characters to the new level cap.
Ideally, you probably don’t want to go all the way to the new level cap, instead opting to get as far as possible before the leveling slows down, then reincarnating and start putting the Karma to use in increasing your base stats. Keep doing this, then testing out the various Rakshasa Dimension area maps, stopping once you reach the final one. Rakshasa V is a special map, as you’re not really going to brute force it on your first clear of it, so you will need to develop a strategy for it. Also, since Rakshasa is the final difficulty, you can begin working on diving into the Item World of the Rakshasa items.