Ahriman

Improves invunerable saves by +1 (to 3++). Confers a 6+ Invunerable if no existing save is present.

Units containing one or more models with the Fearless special rule automatically pass Pinning, Fear, Regroup tests and Morale checks, but cannot Go to Ground and cannot choose to fail a Morale check due to the Our Weapons Are Useless rule. If a unit has Gone to Ground and then gains the Fearless special rule, all the effects of Go to Ground are immediately cancelled.

A Model with this rule must issue and accept challenges. Where there are multiple models with this rule, the controlling player may decide.
Whenever a model with this rule kills a character, roll a D66 and consult the Chaos Boon chart. This boon remains for the rest of the game.

Independent characters can join other units.  They cannot, however, join units that contain vehicles or Monstrous Creatures.  They can join other Independent Characters, though, to form a powerful multi-character unit!  Independent Characters pass Look Out, Sir rolls on a 2+. A unit that contains one or more Independent Characters does not need a double 1 to Regroup if reduced to below 25% of its starting numbres, but instead tests as if it had at least 25% remaining.

Full details in BRB. Rule too long to copy verbatim.

Models with this rule have +1 Ld and Hatred (Space Marines)
This Hatred applies for the following books:
Codex: Space Marines
Codex: Dark Angels
Codex: Space Wolves
Codex: Blood Angels
Codex: Grey Knights