For simplicity, I put the Marks into three categories: Offensive, Defensive and Both.
Mark of Khorne
|He's just so... Khorney!|
This one is pretty obvious in its use: put it on a unit and they'll massacre in combat. This is the only offensive mark we have, i.e. the only mark that is designed purely to cause more casualties but will not prevent you from taking casualties in return.
The kinds of units you want this on are units that will be throwing their weight around in combat. The other important thing, however, is that you want to consider whether or not the unit is tough enough to actually punch enough things for them to do their job, or if they're cheap enough for you to not worry about their survivability anyway, as long as they do enough.
Units like Chaos Space Marines and Cultists with close combat weapons, and maybe Raptors are perfect examples of cheap units that can go punch-for-punch with some more elite units given a Mark of Khorne, while Mutilators and Terminators are some of the more expensive but tougher units. I wouldn't take the Mark of Khorne on Warp Talons personally because it just seems like far too much overkill.
It should go without saying really, but anything that should be shooting is not worth giving a Mark of Khorne, this includes anything with Plasma Guns. If you give a Mark of Khorne to a unit of Chaos Space Marines, make sure they get Meltaguns/Flamers. Rapid Fire weapons are wasted points here.
Mark of Nurgle
I think the main thing really is: when do you take the Mark of Nurgle, and when Tzeentch?
Well, the Mark of Nurgle doesn't improve your save or give you a better chance at stopping your guy from dying after the wound has been caused, instead it simply reduces the number of wounds taken altogether.
In general, the Mark of Nurgle helps against anything that's S6 (S7 if on Bikers or Spawn) or worse by simply lowering the number of saves you have to take. This means that your models with a Mark of Nurgle will generally be very resilient when it comes to walking through a hail of Bolter fire or weathering an Explodes! result from a vehicle.
But what does it not provide defence against? Well, simply put, things like Plasma Guns, i.e. anything with a high enough strength to wound you on a 2+ anyway, which will often ignore your saves as well. In situations like this, the Mark of Nurgle becomes completely worthless.
Fortunately most guns are not Plasma Guns, but unfortunately Plasma Guns will be the bane of your existence.
The kinds of units that benefit the most from the Mark of Nurgle are Bikers and Spawn thanks to their already bloated toughness, which actually means that even Plasma Guns will only be wounding on 3's. Chaos Space Marines, Raptors or anything else without a pre-existing Invulnerable Save also benefit from the Mark of Nurgle far more than they do from the Mark of Tzeentch.
Mark of Tzeentch
|Needs more change to be more competitive|
The Mark of Tzeentch protects you against all the things Mark of Nurgle doesn't, but won't help you at all against the things that the Mark of Nurgle does, so this puts a player into quite the sticky situation... Who do I take which Mark on? When?
Well, first of all the Mark of Tzeentch really is pretty pants on a unit that doesn't already have an Invulnerable save, because a 6++ is just really not worth the points. So that leaves us with the likes of Terminators, Warp Talons, Obliterators, Mutilators and Possessed.
Whether you take Mark of Tzeentch on these units or not depends on a few things:
- Do you want to avoid terrain so you can move faster/avoid Dangerous Terrain?
- Is there little 4+ cover terrain pieces in the area you play?
- Do you need a unit to escort a MoT Lord/Sorcerer?
- Do people in your area spam Plasma/other AP2 weapons?
So the Mark of Tzeentch is a bit more specialised and a far less safe option than a Mark of Nurgle, but it does have its perks in certain places and is definitely worth considering.
Mark of Slaanesh
|"Both" doesn't just apply to Mark uses here...|
The Mark of Slaanesh is, in my opinion, the best Mark overall in the book. The Icon of Excess does play a role in this, but we'll get to Icons in just a moment.
With a +1 Initiative bonus, the Mark of Slaanesh applies only to combat, but is still something worth considering on certain shooty units as well for the Initiative alone.
To understand how +1 Initiative is a fairly potent offensive tool, we must first work out how it's good defensively.
In essence, the Mark of Slaanesh prevents you from taking casualties in the same way the Mark of Nurgle does: by reducing the number of saves you actually have to take. It does this by allowing you to strike before most opponents, which allows you to kill a couple of models in combat before those models even have a chance to strike which therefore means that there are less attacks coming back at you, less attacks hitting, less attacks wounding, less saves you have to take and ultimately less casualties caused to your unit.
But here's where it gets good...
Unlike the Mark of Nurgle which has absolutely no benefits in combat against something like a Dreadnought, the Mark of Slaanesh allows you to hit it with a whole bunch of Krak Grenades before it can hit you, which means that you can potentially knock off the last of its Hull Points before it turns your unit to mush with its S10 AP2 attacks.
So how does it work offensively?
Well, it's mostly in the way it allows you to play your models: more aggresively. Normally your standard CSM wouldn't be able to take on certain units in combat, but because of their improved Initiative they can plow right into the enemy because of the significantly reduced fear of taking too any casualties and losing combat. Furthermore, the unit can only really be defensive if they have some offensive hitting power, otehrwise everything just goes downhill.
But it's also a great offensive stat against the likes of Eldar and Dark Eldar who might otherwise use their Initiative in the same way you'd use yours: to cause casualties before the enemy can strike and thus take less in return. By having a Mark of Slaanesh you now have an offensive bonus against their Initiative 5 units in that you remove their defensive bonus from Initiative, ultimately leaving them with almost no defense and allowing you to strike with all of your force.
The only real downfall of the Mark of Slaanesh is that it only works in combat, but then again, combat is ultimately inevitable in some situations.
Units that work well with a Mark of Slaanesh are any combat-oriented unit, as well as any short-ranged shooting unit (Chaos Space Marines with Plasma Guns for example) because they're generally operating in a 12" distance from the enemy and will likely get charged.
The important thing to remember is that the more damage your unit can cause in close combat, the better it will be defensively as well. Maximise the damage you do, minimise the damage you take.
With that out of the way, it's time for...
Icon of Wrath (Mark of Khorne)
Gives you Furious Charge and rerolls to your charge range. Absolutely fantastic in my opinion. It really just makes the unit better at its role, no more need be said.
Icon of Despair (Mark of Nurgle)
This is probably the worst Icon of the lot. It's cheap, sure, but with the amount of Fearless/ATSKNF/Leadership 10 around it'll hardly have much of an effect ever. However, when and if it does work, it will be brutal to the unit on the receiving end, so it's not too bad if you have a spare few points lying around.
Icon of Flame (Mark of Tzeentch)
A reasonable Icon for a shooty unit and I've seen some people take Mark of Tzeentch just for the Soul Blaze, but I don't think it's that great. It has a place on a unit of shooty Terminators or some Obliterators, but otherwise won't see much play because most of the units it'd be good on aren't that great with a Mark of Tzeentch in the first place.
Icon of Excess (Mark of Slaanesh)
Throw this on the unit and suddenly you've got one of the best defensive kits in our book, beaten out only by Plague Marines themselves. Suddenly your units can whether a storm from a distance and beat face in combat. The best Icon in the codex.