All units in the Daemons of Chaos army have the Daemonic special rule. This includes all the rules detailed below:
Daemonic Aura
All Daemons enjoy the physical and mystical protection of their diabolical masters. Daemons get a 5+ Ward save.
Demonic Attacks
Being corporeal manifestations of Chaos, A Daemon's attacks are magical. This includes any special or ranged attacks they may have.
Daemonic Instability
When Daemons lose a combat they must take a special kind of Break test called a Daemonic Instability test. In multiple combats, each Daemonic unit must test separately. Use the following procedure to take a Demonic Instability test:
Calculate combat resolution as normal and roll 2D6.
Compare the dice roll to the Daemons' Leadership value, taking into account any modifiers for combat resolution. For each point the unit fails its Instability test by, the unit suffers one additional wound. No saves of any kind are allowed against these wounds, including Ward saves, regeneration, etc. If characters are present in the unit, the controlling player first allocates wounds to the unit (up to their current Wounds), then divides the remaining wounds as equally as possible amongst any characters.
Note that an Instability test can be re-rolled if the Battle Standard is within 12" and/or tested on unmodified Leadership if the unit is stubborn.
Example: A unit of Bloodletters (Leadership 7) loses a combat by 4. The controlling player scores 8 on his 2D6 roll. Because combat resolution modifiers count for Daemonic Instability, their Leadership is reduced to 3 for the test. This means that the Bloodletters have failed the test by 5, and so the unit suffers five wounds.
As well as the rules listed above, many Daemons have additional special rules. Most can be found in the Special Rules section of the main Warhammer rulebook. If this is not the case, the effect of the special rule will be printed in the unit entry or there will be a reference to the page on which you can find the rule in question. Some entries refer to Daemonic Gifts.