The original game is great, but the expansion content is a total letdown.
Instead of a coherent somewhat-satirical storyline, the expansion has cliche and juvenile storyline that could be hacked into any other game and fit in just as well as it does in Overlord. Each abyss isn't remotely funny or entertaining.
The main problem is that the expansion doesn't focus on strategy but rather mouse work (probably what they thought was strategy). Overlord had puzzles that required lots of thinking but easy movements so even poor action players could execute the solutions. The expansion does the opposite: the puzzles are pathetically easy and the majority of the work is in moving minions and objects around.
There's also a ton of nonsensical and poorly scripted mini-games like mowing the lawn, breakout, etc. I didn't buy this game to play poorly constructed mini-games with terrible controls. This is the exact reason why I hated the recent GTA games. If they wanted to put an Overlord version of breakout in the game, then make the controls as responsive and easy to use as a standalone breakout game or they should remove it.
I strongly recommend that everyone buy the regular Overlord without this expansion or just don't play the expansion abysses, it will totally ruin your appreciation of the original Overlord. I played through the entire expansion content hoping for something good but there was nothing. In fact, playing through the expansion was so boring and tedious that it felt like I was doing chores.