We’ve seen other Nippon Ichi characters like Marona pop into Disgaea games before, usually as downloadable content. Disgaea Legion Battle takes this a step further by bringing together characters from Disgaea 1-4, Disgaea Dimension 2: A Brighter Darkness, Criminal Girls, Hayarigami, Phantom Brave, Legacista, and Rhapsody into a single game.
The "Legion Battle" part is for a new battle system where up to 15 players can team up to fight a rival legion. GMO developed Disgaea Legion Battle for Android. It’s a free download with additional items available through microtransactions.