There are tales of the Legion of the Damned from all across the galaxy. Just who they are, and where their true loyalty lies, is a matter of much speculation. Some even doubt they exist at all, but the heaps of corpses left in the wake of their awesome, implacable advance cannot be disputed.
A Space Marine army may include a single Legion of the Damned squad as an Elites choice.

Squad: The squad consists of between 5 and 10 Legion of the Damned Space Marines.

Weapons: Bolters. Any model may replace their boltgun with a bolt pistol and close combat weapon at no extra points cost.

Options: One model in the squad may be armed with one of the following weapons: heavy bolter at +5 points; missile launcher at +10 points; or a lascannon at +15 points.
In addition one Space Marine in the squad may be armed with one of the following weapons: flamer at +6 pts; meltagun at +10 points; plasma gun at +6 points.
The entire squad may be given frag grenades at an additional cost of +1 point per model and krak grenades at an additional cost of +2 points per model.

SPECIAL RULES
Apparitions: The Legion of the Damned appear from nowhere, completely unheralded. In every battle they use the special Deep Strike deployment rules, even if the mission does not normally allow troops to Deep Strike. At the end of every Space Marine turn after the Legion of Damned arrive, roll a D6. On a roll of a 1, the Legion of the Damned disappear as eerily as they came - remove the models from the table immediately - victory points are earnt according to their strength when they disappear (if only half of them are left, your opponent scores halt the unit's points value in victory points, etc).
The Legion of the Damned may not be joined by an independent character, they always fight alone.

Fearless: It is debatable whether the Legion of the Damned are truly alive and have any real minds. Legion of the Damned automatically pass any Leadership-based tests, including Morale checks, that they are required to make.

Terrifying: Any unit which loses an assault whilst fighting the Legion of the Damned suffers an additional -1 modifier to its Leadership for the subsequent Morale check.