I have picked up the book.
First up, Be'lakor now has Khorne, Tzeentch, Nurgle and Slaanesh as faction keywords. So you could now use him in a mono god detachment or army (there can be no other Daemon Prince in the same detachment though. He hates them to such an extent that he get a full re-roll to hit AND to wound against them).
Now for his Disciples. There are a lot of restrictions. The perhaps biggest one right now (for matched play) is that the daemons and mortals don't share any Faction keyword other than CHAOS unless they are all dedicated to the same god. So if you want to run with CSM they must all have the same mark (to have a Faction keyword in common with Be'lakor) and you may only bring one other unit of daemons besides Be'lakor (as you can't select another daemon unit dedicated to the same god until you have taken one from each god). If you want to focus on Daemons you can't bring any mortals at all. This might be an oversight from GW (the Disciples of Be'lakor keyword might have been intended to be a Faction keyword, but right now it isn't). This is obviously only a problem for matched play.
For the other restrictions if you want to play Disciples of Be'lakor...
Be'lakor must be your Warlord.
You ARMY may not include any:
- Greater Daemons
- Daemon Princes (other than Be'lakor)
- Daemon Engines
- World Eaters
- Thousand Sons
- Death Guard
- Emperor's Children
- Chaos Knights
- Titanicus Traitoris
- Khorne Berzerkers
- Rubric Marines
- Plague Marines
- Noise Marines
In addition to that you cannot include more Chaos Cultists units than Heretic Astartes Infantry units (Characters don't count). Also you can't take a second daemon unit with allegiance to one god until you have taken one unit each from the other three gods.... then you can't take a third daemon unit with allegiance to one god until you have taken two units each from the other three gods.... and so on. Be'lakor doesn't count. All the restrictions mentioned also applies to units you summon (so you can't summon a Greater Daemon or bypass the "unit from each god" rule).
What you get:
- Objective secured for all Troops
- Daemons can still use Stratagems, Relics and Warlord traits as if they were in a Chaos Daemons Detachment.
- The same is true for Heretic Astartes (though they can't use any Legion specific stuff).
- You don't get a Daemonic Locus (obviously, as you can't fulfill the requirements) or Legion traits. Instead you get Locus of Shadows (for daemon characters) and Disciples of Shadows (for CSM). Both abilites seems good (both include minus 1 to hit if more than 12" away as was shown in the previews).
- You also get access to Disciple Stratagems and Be'lakors own Psychic discipline. Both include good stuff (such as psychic powers that teleport units or make a unit untargetable with shooting attacks or the strat that removes all damage from an attack).
Oh... and the warlord trait part is only for Crusade I guess as neither army can take extra warlord traits and Be'lakor is always you warlord.
If you are looking to run a mixed Daemon army, this seems to be the best option (as you actually get something while doing it). The restrictions might seem very limiting but you still have a lot to choose from. And if you don't want to play around with all the gods at once you could use Be'lakor in your normal daemon army instead (he will even benefit from the Daemonic Locus now that he is no longer Undivided). Haven't looked too much into what you can do for Heretic Astartes yet (but it probably involves the Mark of Slaanesh). Feels like an interesting army though, but not top tier if that is what you are looking for.