Black Legion: Using those new allied FOC slots
The Black Legion book has introduced some unique opportunities for Chaos Space Marine players, largely due to the extra Force Organization Slots offered by being able to ally as Battle Brothers, much like the Tau Farsight Enclave. Most likely, if you are playing Chaos, you would rather use Chaos Space Marines as your main force, with allied Black Legion, to avoid that hefty VOTLW tax, but the real question is, what do we do with this new found freedom?
Having a 3rd HQ makes a big difference. Not only can you get more characters to make your units fearless, but it's also the only place where you can get the new Black Legion Artifacts.
Chaos Lord - This is the budget choice, but shouldn't be ignored. If you're running an HQ heavy CSM already, this could yield you a 3rd Nurgle Bike Lord, or another killy Khorne Lord on Juggernaut (with Spineshiver Blade), or at the very least, an inexpensive way to hand out fearless to one more unit.
Chaos Sorcerer - My personal favorite choice, though largely due to the Last Memory of the Yuranthos. Throwing him on a bike as a level 4, rolling in telepathy for 3 spells, is going to get you some nasty powers. The ability to cast both invisibility and hallicunation is amazing, but also you could cast one of those as well as a 12" nova of doom. The versatility that being a level 4 psyker gives you is really enjoyable, the only downside being that this easily ends up as a 180 point model with 2 wounds, so guard him well!
Daemon Prince - My least favorite choice, but an interesting one nonetheless. Since he can take an artifact to make him into an Eternal Warrior, he becomes much more survivable for fighting enemy monsters, and not going to get instagibbed by S10 smashing beasts or rail guns.
Well, there is good news and bad news for the Troops section. The good news is that you could take allied Chosen as troops, the bad news is that like nearly anything else, you'll be paying the VOTLW tax. Fortunately though, there's another alternative! Chaos Cultists! For a mere 50 points, you can get yourself 10 cultists and avoid spending any points that you wouldn't be spending already, it's not like people ever run Chaos Marines with only 2 troops, let alone without any cultists. Since you're taking them anyways, you may as well take them as allies!
Remember one thing though, allied Cult troops will ALWAYS be elites, they are only moved to Troops in your primary detachment.
Sadly, chaos has very little to offer for the Elites section. Imperial Armour Apocalypse at least gives us Giant Spawn, though I think it's doubtful that anyone will want to run 4 elites of any variety in a Chaos Marine list.
A complaint I've had about Chaos Marines from the beginning is that their Fast Attack section is far too impacted, well that's all changed!
Chaos Bikes - Chaos bikes have to pay the pesky tax on VOTLW, but fortunately it's only 1 point here. I would still avoid taking them as allies, and instead using the allies slot for a Heldrake or Chaos Spawn, since they don't need to pay the tax. That said though, freeing up a slot from your main FOC means that you could fit in that extra unit of bikes you always wanted. They're cheap, efficient, and with Mark of Nurgle, a total pain in the neck to kill.
Chaos Raptors - Like Chaos Bikes, I'd take this in your main FOC, especially since their VOTLW costs two points per model instead of one! Before Chaos Raptors were often overlooked because there was just too much competition for the slot, but with one more Fast Attack, I think these guys might actually be viable. With Mark of Nurgle, they become the same cost as a naked chaos biker, equally as resilient, and can swoop over terrain without problem. If you want to keep them cheap, that's possible too, and you can also get a mighty 15 per squad, with 2 meltaguns for extra fun.
Heldrake - The first thing people said when the Black Legion supplement came out is "OMG 4 Drakes!" While that is possible, I've found diminishing returns on taking more than 2 of these suckers. That said, taking them as allies is perfect! It frees up a FOC slot in your main detachment, and doesn't have to buy Veterans of the Long War.
Chaos Spawn - Like the Heldrake, Chaos Spawn can't take VOTLW, which makes them perfect in an allied detachment. 2 units of Chaos Spawn, with characters, plus 2 Drakes is going to make a for a disgusting and fast army.
Chaos Havocs - Taking VOTLW isn't really a problem for Havocs, most people tend to take them with that upgrade anyhow for the leadership, so taking more Havocs for allies really isn't incurring any extra cost anyhow. Imagine an army with 4 units of havocs with Autocannons, that's a lot of firepower!
Obliterators - Like Havocs, most people take Obliterators with VOTLW already (at least in units of 2-3). If you really want to build a shooting heavy army, it's entirely possible to now take a dozen of these nutters. Slap Mark of Nurgle on them and suddenly you have an extremely hard to kill wall of fire pouring across the battlefield. You could even attach a chaos lord and march them across the board, once you get close charge in and crush everything with your power fists.
Maulerfiends - Having a 4th Maulerfiend in an army with 2 units of bikes/spawn is probably my favorite thing about the Black Legion supplement. With these extra slots, you really can make a fast as hell army, just imagine it... 2-3 bike characters, 2 units of T6 bikes/spawn, and 4 Maulerfiends all within charge range on turn 2, if you go first, that's going to be a very unhappy tau player.
Predators - I had actually just thought of this, but Chaos Predators are dirt cheap, so being able to add another one to an army couldn't possibly be bad! For a rather low investment, you could even get 4 of them with Autocannons, Heavy Bolters, Havoc Launchers and Warpflame Gargoyles, just to dish out the maximum dakka for just under 450 points... Gross!