Underlord was a fun entry to the series. More and more it's hard to think of these as individual books as opposed to one larger story - I don't mind this at all as a reader, however it makes reviews more difficult. 

Overall Underlord had a solid self-contained narrative. A number of previous foreshadows finally paid off, while also developing the storyline further. Underlord brought us into the larger world in a much bigger way than we've experienced previously, but didn't do it in an overwhelming way. 

Underlord also probably brought the deepest character work that we've seen so far. Which was a welcome addition to the series. 

A solid entry to a solid series.