I have always loved the secrets behind the Legion of the Damned, so of course, I had to pick up the "Codex." I'll get the first bit out of the way early - it's a Codex in the same way as the Inquisition Codex is. By that I mean that there were no new units introduced, just new ways to field existing units either as a standalone force or as allies.
There is a little bit of fluff that I have not read yet that I'm looking forward to, but game wise, I don't expect this book will have any major impact.
The LotD formation is, quite simply, one mandatory Elite unit (a regular squad), and up to three additional squads. If they are the primary, all LotD units from the detachment are scoring.
The book includes an allies matrix, which indicates LotD are battle bros with all Imperial humans, allies of convenience with Eldar, Desperate allies with Tau and Dark Eldar, and otherwise, come the apocalypse.
If you take the Legion as a primary, one sergeant is nominated as the warlord and rolls on their own table. The possible results include giving his unit the Soul Blaze USR, a negative modifier to fear checks taken by models in combat with the warlord, Armorbane with bolter weapons for his unit, Feel No Pain for his unit, allowing the controlling player to decide when the warlord and unit will arrive from reserve, or Preferred Enemy.
Finally, there is a new item that a sergeant can take for the same cost as a legionnaire with a meltagun. This item gives the unit Feel No Pain, which is boosted by 1 for a player turn for each enemy unit that fails a morale check within 12". Additionally, a random model from the unit that failed the check is removed from play.
The Warlord Traits are surprisingly good, even if they're on a less than sturdy platform with pretty limited options. The item is a little meh, since the special bonus is unlikely to happen very often, and it doesn't last. Feel no pain for the unit is still nice.
Finally, the codex contains several themed missions. Along with the fluff I'm looking forward to giving these a good read through.
But in summary, rules wise, there's not much that I think is new, game changing, or particularly unexpected from this book.