Daemon Weapons
Forged in the flames of darkest evil, Daemon swords, blades and axes are amongst the most potent of weapons. Each blade contains the bound essence of a denizen of the Warp, whose terrifying powers may be employed by a bearer strong enough to gain mastery over it. But even the strongest may be forced to pay a price, for should the blade still thirst for souls when all enemies lay slain, then it may just turn its attentions on the one who wields it.
Unique Weapons
Daemon weapons are much sought after weapons. You may only include a single Daemon Weapon in your army.
Just because a weapon is described as a blade or sword does not mean it always takes this form. Modellers should feel free to represent a Daemon weapon how they wish, as long as it looks suitably impressive and they inform their opponents what it is.
Some Daemon weapons are two-handed. This is specified in the weapon's description. A model wielding a two-handed weapon may not use an additional close combat weapon. There is no additional strength benefit from using such a weapon beyond the normal abilities of the weapon.
The costs and rules for individual Daemon weapons can be found in the Gifts and Wargear section, and in the Books of Chaos. Some Daemon weapons may only be used by a model with a particular Mark. These will be found in the relevant Book of Chaos.
Mastery
In any player turn that the bearer inflicts at least one casualty on the enemy with a Daemon weapon, the Daemon may gain enough strength to resist its owner. This battle of wills can be draining or even fatal for the wielder. Make a Leadership test at the end of the turn. If the test is failed the wielder suffers a 'Perils of the Warp' attack. If the test is passed or if the wielder survives the Daemon's attack he re-asserts his mastery and continues as normal (at least until the Daemon weapon rebels again!) Note that this is a Leadership test, not a Morale check, so characters who are Fearless must still take this test.
Errata — Chapter Approved: Codex: Chaos Space Marines Q&A (v3.0): The Mark of Chaos Undivided does not grant a re-roll on a failed Mastery test. A special character's own Daemon weapon still counts towards the army's 0-1-per-army limit if the Armoury defines that weapon as a Daemon weapon (for example Abaddon's Drach'nyen does, but Fabius Bile's Daemon-forged Rod of Torment does not, since it isn't defined as a Daemon weapon as such).