First: a little background into the actual Halo series of video games. The Halo games are Hollywood action movie blockbusters. They are not intended to provide compelling characters (the main character is basically mute and faceless). And the setting is aimed at having the broadest possible appeal. Therefore, instead of creating new and unique aliens, Halo casually repurposed the aliens from the popular computer game Starcraft. Given that Halo is about a space marine running around gunning down evil aliens all this is fine. But nothing about Halo is particularly deep or interesting (to me anyways).
Bear sidesteps a lot of the problems of writing a Halo book by setting Cryptum tens of thousands of years in the past. Because of course Halo has that old hoary trope of ancient god-like aliens who left ruins filled with a secret, dangerous power. Cryptum is something of a novel premise then: it is set at the height of the galaxy-wide hegemony that left its moldering mark upon the universe of Halo. It's actually something that I have never seen attempted before. For a good reason I think. The big draw of the cliche is the mystery- the feeling that the full story of the bizarre alien artifacts and the mighty power that made them can never truly be known. Part of the allure is the power of the civilization that made devices so incredible that they have endured both the passage of time and their own creators. Setting a story in a civilization so advanced and powerful is problematic. Likewise, coming up with a believable apocalypse is tricky. Then there is the problem of scale, how can a character get ringside seats to the end of days and still get across the size of the event. Finally, the main character and his people are doomed, totally doomed. The reader knows that no matter the heroics of the characters they will fail.
Not appearing in this book!
Greg Bear handles these challenges with a varying degree of success. The Forerunner's (such a name for a doomed race!) culture is intriguing but its hard to get a handle on their exact capabilities and day to day life. Rather than spend the book world building, the plot races from location to location providing the information necessary for conflict; but leaving the setting weirdly sterile and blurry. I suppose it was too much to ask for a sociological breakdown of super aliens in a franchise tie in book though.
The story is (in what I think is a pretty bold choice) a coming of age story. It follows the journey of a young adventurous Forerunner as he... gets swept up in a giant crisis? Because of that damn hazy setting its hard to really feel the full force of the threat of the disasters menacing the Forerunners. The reader isn't invested in the Forerunner civilization- how could we be invested in something we know so little about. And where are the humans? Humanity is basically window dressing in this story. And where is the Halo franchise in all this? Well there is powerful robo-armor and blue lady AIs that live in the armor. ...just like Halo! The two evil aliens from Halo make a cameo appearance. Beyond that Cryptum is practically its own beast.
If all this kind of sounds like a mess that's because Cryptum is a mess. It can be an interesting read at times. Enjoyable even. But ultimately its just too muddled for me to really recommend. I guess if you are a huge Halo fan you could read it. Halo Cryptum receives only 5 of the original 12 Halo rings.