Taking the Legion of the Damned
Legion of the Damned Detachments.
The Legion of the Damned can be taken as a primary detachment or as a special form of allied detachment called a Legion of the Damned detachment.
As a primary force you can take can take 1-4 units of Legion of the Damned units, 1 of which is required and up to 3 additional counting as optional. If you do this, you can take an allied Legion of the Damned detachment. If Legion of the Damned are taken as a primary detachment, then all Legion of the Damned units in your primary count as scoring units.
As an allied force, like the Inquisition, the Legion of the Damned works outside the perimeters of the foc, allowing players to take a primary, allied, and legion of the damned detachment.
Allies
Here is who can ally with the Legion of the damned.
Come the Apocalypse (cant ally)
Chaos Daemons, Chaos Marines, Necrons, Orks, and Tyranids
Desperate Allies
Dark Eldar and Tau
Allies of Convenience
Eldar
Battle Brothers
Everyone Else, including the Inquisition
Warlord Traits
They also come with warlord traits, which really only effects you if you take the Legion of the Damned as a primary detachment. If so, pick one Sergeant as your army warlord and roll on the chart.
Inferno of Vengeance: Warlord's unit has Soul Blaze
Aura of Fear: -1 cumulative to leadership for any unit in base contact with the Warlords for each Legion of the Damned unit on the board, beyond the first.
Ethereal Bolts: Each bolter and bolter weapon have armour bane. Whoa
Spectral Bulwark: Warlord and unit have Feel no Pain
Never Too Late, Never Too Early: Can choose when to come in from reserves for the Warlords unit
Retribution made Manifest: Warlord's unit has preferred enemy.
Artifact
There is also a single artifact called the Animus Malorum. This thing can be a beast, as it gives the Feel No Pain, and each time an enemy unit fails a Fear, Pinning, or Morale check within 12" one random model in that unit is removed from game with no saves. Each time a models is removed this way, the units feel no pain is increased by +1 to a maximum of 2+.
This mini-dex really adds to the game one of the most mysterious of groups in the game. It will be first on my list to read through the background, as what I have read looks very interesting regarding the exploration of the Legions origins.