A Crozius Arcanum is a mace or staff that is used as both a melee weapon and the primary badge of office of Space Marine Chaplains. This duality of purpose presents no philosophical conundrum for the Chaplains, as for them battle itself represents the most valuable form of worship of the Emperor of Mankind. It is normally shaped in the form of either a staff or a mace, topped by the double-headed Aquila of the Imperium or a winged skull that represents the Emperor's sacrifice. A few versions use iconography from the Chapter they belong to, such as the blacksmith's hammer-version used by some Chaplains from the Salamanders Chapter. Chaplain Ortan Cassius, the Master of Sanctity (chief Chaplain) of the Ultramarines Chapter, has fashioned his Crozius Arcanum in the form of a Tyranid head, in remembrance of Hive Fleet Behemoth's costly invasion of their homeworld of Macragge during the First Tyrannic War. Incorporated within the shaft of a Crozius is a powerful gravitic energy field capable of disrupting matter much in the same way as a Power Weapon. The Crozius is the badge of office of Chaplains and is normally used in many Space Marine ceremonies and rituals across a vast array of Chapters. To show their faith in the Emperor of Mankind, the Crozius, along with the Chaplain, is most likely to be adorned by Purity Seals and other tokens of faith.
The Crozius Arcanum is still used by the corrupted Dark Apostles of the Word Bearers, which is the only Chaos Space Marine Traitor Legion still to employ the use of Chaplains. However, in the hands of the Dark Apostles, the weapon is known as an Accursed Crozius, now a symbol of the Dark Apostles' devotion to the Ruinous Powers of Chaos in the form of the Chaos Star (representing the Eight-Fold Path), rather than a sacred emblem of their reverence for the Emperor. Twisted and warped from the pure Crozius Arcanum of Loyalist Space Marine Chaplains into something hideously blasphemous, an Accused Crozius mocks the Imperium with its mere existence. Not only is an Accursed Crozius still a potent weapon, it also signifies that its bearer is blessed by the Chaos Gods, enjoying physical protection as well as a closer connection to them, and ability to summon the Daemons of the Warp.
At the time of the Horus Heresy, most of the Chaplains of the Traitor Legions resisted the lure of Chaos and were slaughtered by their fellow Space Marines. Only the Chaplains of the Word Bearers were as possessed of the same fervent desire to find entities truly worthy of worship as their Primarch Lorgar, and so they willingly followed him into devotion to Chaos Undivided. To prove their dedication to faith in the Ruinous Powers, the Chaplains corrupted their once-sacred Crozius Arcanum, a symbol of the Imperial Truth that they had now abandoned. Even worse, some became home to vengeful Daemons that sought not only the violence that surrounded a Dark Apostle, but also the endless supplications of the Apostle's mortal followers. In battle, an Accursed Crozius serves not only as both a weapon and mark of a Dark Apostle's rank, but also a powerful link between the bearer and the entities of the Warp itself.
- Black Crusade: The Tome of Excess (RPG), pg. 37
- Codex: Black Templars (4th Edition), pg. 46
- Codex: Blood Angels (5th Edition), pg. 43
- Codex: Blood Angels (4th Edition), pg. 8
- Codex: Blood Angels (3rd Edition), pg. 19
- Codex: Chaos Space Marines (3rd Edition, Revised Codex), pg. 43
- Codex: Space Marines (5th Edition), pp. 58, 97, 133
- Codex: Space Marines (4th Edition), pp. 18, 23
- Codex: Space Marines (3rd Edition), pg. 40
- Codex: Space Wolves (5th Edition), pp. 23, 50
- Codex: Space Wolves (3rd Edition), pg. 29
- Codex: Space Wolves (2nd Edition), pp. 34, 39, 48, 76-77
- Codex: Ultramarines (2nd Edition), pg. 73
- Deathwatch: Rites of Battle (RPG), pp. 107-108, 140
- Deathwatch: The Jericho Reach (RPG), pg. 78
- Horus Heresy: Collected Visions
- Index Astartes II, "For the Emperor - Space Marine Chaplains"
- Index Astartes IV, "Dark Apostles - The Word Bearers Space Marine Legion"
- Warhammer 40,000: Compendium (2nd Edition), "Chaplains & Commissars," pg. 22
- Warhammer 40,000: Wargear (2nd Edition), pg. 8
- White Dwarf 311 (US), "Chosen of the Emperor" by Nick Kyme, pp. 22-23
- White Dwarf 270 (US), "Index Astartes: Dark Apostles"
- White Dwarf 208 (US), "Faith and Vengeance: Space Marine Chaplain Background and Tactics", pp. 78-86
- False Gods (Novel) by Graham McNeill
- Fulgrim (Novel) by Graham McNeill
- Tales of Heresy (Anthology) edited by Nick Kyme and Lindsey Preistley, "Scions of the Storm" by Anthony Reynolds
- A Thousand Sons (Novel) by Graham McNeill
- Battle for the Abyss (Novel) by Ben Counter
- The First Heretic (Novel) by Aaron Demski-Bowden
- Aurelian (Novella) by Aaron Demski-Bowden
- The Blood Angels Omnibus by James Swallow
- Chapter's Due (Novel) by Graham McNeill
- Daemon World (Novel) by Ben Counter
- Know No Fear (Novel) by Dan Abnett
- Word Bearers Novel Series:
- Dark Apostle (Novel) by Anthony Reynolds
- Dark Disciple (Novel) by Anthony Reynolds
- Dark Creed (Novel) by Anthony Reynolds, pp. 9, 111, 184-185
- Dawn of War - Dark Crusade (PC Game)