Just to briefly explain the setup to Invincible newbies, Mark is the son of the world's most powerful hero who turns out to be an alien villain. As both himself and Invincible, Mark has a lot of growing up to do and a lot to deal with in terms of new enemies, new friends, and complicated relationships and it's all set in the Image Universe which allows for some fun teamups. And this comic is fun in a big way.
That said, this 12th trade paperback really darkens down the tone and it's the grimest installment so far. It begins with about 20 hostile Invincibles from alternate worlds invading Earth, one of whom specifically wants to kill Mark's girlfriend. In the aftermath of this disaster, matters get even worse with the arrival of Conquest, possibly the strongest of Mark's father's people. Now while I'm not one for issue-long super-hero fights -and this lasts for nearly three of them!- this is done really well and it's the darkest most violent and horrific installment of Invincible yet.
If you haven't tried Invincible before, don't start with this. Go and get the first one. If you have then you'll hardly be surprised to learn that this comes highly recommended.