Did you play the first Overlord? Then you already know what i'm going to talk here.
Overlord II had the same gameplay of the first game, a little bit improved, letting you control the minions more smoothly and so. It has some clearly better graphics, but not that better. The plot? well, the plot is almost "creative-less". You start as the original overlord's son(ok, that's not bad), later on the game you become the Overlord, and then you have to conquer all. That is the basic concept of the first game(you don't start as a son neither as a boy though) and the second one. Now, let's look deeper into the second game. Just to make clear, i'll be making a comparison between the first game and the second.
The first's plot was good, immersive, creative and fun. The second's plot... is "plotless". It's just that way that it's almost random. The first one's bosses were great fights, all bosses where diferent and you probably won't feel bored in any of those fights. Now, in the second game, the bosses' fights are somewhat repetitive and, well, there are only three bosses(major fights to be more specific), while in the first there are more bosses and a much better ending boss(belive me, the second's last boss is truly lame, i'm not going to say what it is to not show any spoiler :) ). Also, the music in both games is really good. Although, the first one has a bigger variety of musics.
There isn't much more to say about it. Codemasters was only milking the Overlord franchise this time(see the number of the games this time), leading to a bad game(comparing to the first, it is). If you haven't played any of the two, i suggest you play the first one for better plot, fights and fun jokes. If you did play Overlord, play it again instead of buying the second game. In case you're kinda bored, and doesn't have anything to do, buy Overlord II and i hope you find it more fun than i did.