Never actually played one of these, but it's a Tactical RPG series isn't it? Totally up my alley if it is, can't believe I never got at it before. Is this one a sequel, or is independent from the others storywise?
Each of the mainline numeric Disgaea titles have a self contained story. However there are cameos from previous games in the series. However none of these cameos ever reference the plot or story in the game they come from, so they are included as a bonus.
You can start with Disgaea 5 and can easily catch up with the general premise this series is all about.
Basic plot line is there is bunch of Netherworlds all throughout the Netherverse. A bunch of different worlds that have a single ruler in charge called the Overlord.
However it's mainly very light hearted and comical.
Most of the games have three shared worlds. The Human World (usually some version of Earth), Celestia (Where Angels live), and The Netherworld which is basically Makai or the Demon World where chaos rules and opposites of laws exist.
Using Disgaea 3 for example which takes place in a Netherworld School setting, the main character is an honor student who cuts class, never does his homework, and generally does the complete opposite of what a REAL honor student does. In this sense the Netherworld is basically where opposites make sense and wrong is right and so forth.
Unlike serious Strategy RPGs that use mechanics such as permanent character death or complex and mature story writing. Disgaea is more about being ridiculously over the top.
Character levels max out at 9999, and Attack and HP values can go into the millions. The only thing Disgaea takes seriously is how ridiculous can they make the current game more crazy than the last.
Download the demo on PS4, give it a try. If you like it, Your save transfers to the full game. If you don't like it, that's fine and I tried my best give you a fair description on what it's all about.
If you have any questions on the Disgaea series, I will do my best to answer any of them.