“Codex: Legion of the Damned allows you to add Legion of the Damned squads into your Warhammer 40,000 army, or field them as a detachment in their own right. It includes extensive background on the mysterious Legion of the Damned, detailing the times they have appeared to fight alongside the armies of the God-Emperor. Also inside you will find rules for Legion of the Damned units, as well as three new missions, an artefact and Warlord Traits and, all unique to these legendary warriors.”
The artwork is fairly awesome and the background should prove interesting to any fluff nuts out there. As the blurb above says this allows you to use them in your armies as allies or indeed make them a force in their own right. I think they are some of the coolest looking miniatures out there at the moment but they obviously have the road block of being fairly expensive.
There is of course nothing to stop you making your own Legion of the Damned using normal Space Marines alongside plenty of skulls and flames.
Will you become one of the Damned?