To most citizens in the 41st century, daemons are to be feared - outsiders constantly attempting to force their way into their very minds summoned from the Warp by cultists and heretics. What of those who stand guard against them: the Inquisitors of the Ordo Malleus and Space Marines of the Grey Knights? This book seeks to tell you everything that you might want to know about them, including rules for playing them, and other ways of involving them in your game.
This being Dark Heresy, the main focus is of course on the Ordo Malleus, the Inquisitor-led body whose job it is to hunt down sorcerers and the daemons they would summon. In Chapter 1: The Ordo Malleus we can read a wealth of information about the history of the Ordo, about its holdings in Calixis sector, and even meet some of the notables amongst the leadership. Much of its history is shrouded, the nature of the threats they face being held so terrible that records are routinely destroyed. Being a pretty big organisation, it has various sub-groups which you can also read about. The threat may be universally acknowledged, how to deal with it has attracted many opinions, all held fervently by their adherents. Of course, there are frequent notes as to how to play certain individuals or where different facets of the Ordo may fit into your game - but just reading through this material spawns plenty of ideas.
Next, Chapter 2: Hunter Careers turns to more game mechanical material with new options for Acolytes including the Banisher career, individuals capable of battling daemons face to face. There are also new background packages and alternate career ranks, optional routes of progression which bring both benefits and disadvantages to an Acolyte wishing to pursue them. There are some fascinating options here.
To aid them we find the resources of Chapter 3: Malleus Armoury - a collection of ancient and powerful weapons and much more to support the fight against evil. This extensive catalogue includes ranged and melee weapons, armour, force fields, gear, tools, drugs and ammunition, as well as a select array of 'anointed weapons' - holy relics that have legendary status. However, there's detailed information on how to make them too.
Chapter 4: Daemonic Incursions looks at the minions of the Chaos Powers. It's a primer for the GM on how to harness them to good effect within a campaign, providing loads of advice and examples. There are plenty of adventure seeds here too - this is a chapter best left for the GM's eyes, or at least one that players need to keep separate from their in-character knowledge.
Finally, Chapter 5: The Grey Knights provides background information and the rules you'll need if you want to play one of the Emperor's chosen warriors, the Grey Knights Chapter of the Adeptus Astartes. These are the guys you call in if the Ordo Malleus do not manage to contain a daemon threat, providing the overwhelming force necessary to prevent the destruction of an entire world - or more. Now, when the original concept of a role-playing game for Warhammer 40K was first proposed, a lot of people thought immediately of the iconic Space Marines and Dark Heresy must have been a bit of a surprise with its focus on Acolytes combatting heresy rather than Marines thumping anything that moved. Here at last are the secrets of those ultimate warriors, the Grey Knights, complete with information to create Grey Knight characters or NPCs. They are rare, but potent and deadly. It takes a special situation indeed for any Inquisitor to think of sending for one.
This all makes for a fascinating read, and empowers you to unleash the powers of darkness into your campaign. This, children, is why you never ever tamper with heresy... lest a daemon come, with the Ordo Malleus and possibly even a Grey Knight hot on its heels!