Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

I was curious about the black legion/crimson slaughter supps, are they worth getting and what is special about them outside of the normal codex?

I liked the Crimson Slaughter one, mostly because of cool wargear, but more than that it is quite meh. I thought it was worth it, but it is only a couple of Warlord Traits, some wargear, minor unit changes, ruleswise. There is of course some fluff in it and some pictures too.
Basically:

I did not like the Black Legion one, but it is basically the same thing.

Well the crimson slaughter is great because it makes possessed actually usable, and lets you take a possessed HQ choice. If you like possessed, this supp is just the best thing ever.

Black Legion is a great supp because it lets you take chosen as troops without taking Abaddon the Despoiler. So if you like chosen I guess this one is for you.

Other than that, both supplements have lots of cool Chaos Relics to buff up your HQ choices to be a little more viable. My favorite was the balestar, allowing me to finally take a sorcerer with divination.

Crimson slaughter has some interesting relics.
And chosen with prefered enemy rock.
Possessed are usable but still not worthwile as a core element.