+ GW's Apocalypse book is the official rulebook, with rules, superheavies, and formations. That's the one you need, since it has the core Apocalypse rules. Also check the Apocalypse FAQ on the Games Workshop website, as Apocalypse was written for 4th edition and the FAQ has the rules patches needed to play it in 6th edition. + GW's Apocalypse Reload is a softcover expansion, with more units and formations. I believe that it is now OOP. + Forgeworld's Imperial Armour Apocalypse Second Edition, and Imperial Armour Apocalypse II are, like Reload, expansions that offer more units and formations.
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