Overlord 2 adds much more to the Overlord formula, and is for the main part a much better game than the first.
In Overlord 2 you play as a new Overlord who you find to be the son of the old Overlord, and you're in a time period where a glorious empire (comparable to the Roman empire) has rose up, and is exterminating all the magical creatures in the land (which includes you), so of course your main goal is to bring down this dreadful empire; the story for the main part isn't too heavy, but there is a couple of plot twists in there -one significant twist right before the final boss-, but besides that the story isn't too heavy so you won't have to spend to much time thinking about it.
The setting has changed aswell for Overlord 2, instead of it having a a very fantasy-like setting such as in the first, you now have a more realistic, real world-like setting in contrast with the first, as for which one I like better, I can't say. But probably the best thing about Overlord 2 is how much it adds to the formula, they introduce many new things that you can do with you minions in Overlord 2 such as ride mounts (such as wolves, or salamanders), steer boats, and it also gives the Overlord the ability of possessing a minion so you can play as a minion which is very fun; though I should say that the part of your minions steering boats was only in one part and basically non-existent in the rest of the game and I think they should of added more of that.
They also added the ability of dominating or destroying towns in Overlord 2 -there's 2 towns that you can dominate or destroy in the game- which once you destroy their governor you have that choice of domination or destruction, domination means that you brainwash all 100 citizens of the town and put them to work making armor,weapons, and lifeforce for you, and destruction means that you kill the 100 citizens in the towns and get quite a bit of gold, lifeforce, and the like right away, the choice is yours.
And though the original Overlord was only released 2 years ago, the graphics have been greatly improved for Overlord 2, which is always a nice addition. And of course the trademark humor comes back in Overlord 2, yes the dark, twisted, and sadistic humor is back, and it really is hilarious, there is many moments that will leave you either laughing or smiling, and that's something I'm glad they didn't change. So I think Overlord 2 is really a step up from the first (and the first was fantastic aswell!), and I would recommend it to anyone who wants have a fun, unique, gaming experience that they won't forget.