Each book in the War Zone Octarius series presents new rules, army lists, and ways to play. Book 1: Rising Tide introduced the Octarius sector, and the huge battle going on there between the greenskin Ork hordes and the hivemind biomass army of the Tyranids, along with a new campaign to play through and new army list supplements for the Tyranids, and the Imperial Guard. This time, in Book 2: Critical Mass, it's the turn of the Orks to get some new rules. You don't need Book 1 to use Book 2, but having both will give you access to the complete story and new rules introduced through the series. To use either of the new books, you will need a copy of the Warhammer 40,000 Core Rules.
