A free to play event for the Switch provided you have Nintendo Switch Online membership to see if you like the game - the purpose is of course to get you interested in the franchise for the launch of Disgaea 6 in summer 2021.
If you already know you like it and want to buy it cheap, it is on sale until 20 September.
From the website:
Disgaea 5 Complete is a strategy RPG that offers hundreds of hours of over-the-top, award-winning gameplay, and marks the first time the core game is available on-the-go. The game includes all 8 bonus scenarios, 4 fan-favorite characters, and 3 character classes that were originally DLC in the PlayStation®4 release of Disgaea 5: Alliance of Vengeance.