The Pink Horrors of Tzeentch fill the role of long ranged attackers of the Daemon army. It is very important to note that they are physically the weakest of the troop choices; even the Nurglings are tougher than these guys in a fight but all this means is that they're not meant to be in a fight. They have a WS, BS, S, T and I of 3, 1 wound, 1 attack and Ld 7 so they really don't have a great stat line for a basic cost of 90 points and at first glance they're not even that well suited to being a psychic shooting unit as they're meant to be. Looking at those stats you'd be forgiven for thinking that they're just generally a rubbish unit but the Changer of Ways is not known for deception for nothing and it's their combination of special rules that take them from lame to pretty good at what they do. The first of these rules is the rule that really defines them and that's the Daemon of Tzeentch rule which grants two effects; firstly that the Horrors are able to re-roll any saving throws of 1 and the second is that for the purposes of Psychic tests all Daemons of Tzeentch gain +3 to their Leadership value so they effectively have Ld 10. Again this is only for psychic tests but if all goes to plan that should be the only reason they roll for Leadership anyway.
Horrors are the first unit under the 6th Edition to use the Brotherhood of Sorcerers rule and what this does is make the unit as a whole a Level 1 Psyker, is able to use the leadership of a character that is attached to the unit instead of the units (but can't use the leadership of Independent Characters) and any Perils of the Warp are suffered by the unit Character or a random model if there isn't one. In combination with this rule is the unique Magic Made Manifest rule that changes how many Warp Charges the unit generates; a basic unit of 10 generates 1 Warp Charge, 11-15 generates 2 and 16-20 generates 3 Warp Charges. This doesn't change their Mastery Level so no matter how many charges the unit gets they are still only a Level 1 Psyker and still only get one Psychic ability.
Another interesting rule that I think was created as a nod to the back story of the Pink Horrors is the Blue Horror rule which states that for every Pink Horror killed in close combat a counter is placed next to the unit. At Initiative step 1 an enemy that is/was in combat with the unit takes a s2 AP- hit per counter. This is a nice touch that incorporates the story and might possibly give an assaulting unit second thought about attacking them but in practice I doubt a s2 hit is going to really scare anyone into not killing them.
As with all the Troop choices (except the Nurglings) the Horrors can take an Instrument for 10 points and an Icon for 10 points and as with all Icons the Horrors can upgrade theirs to the Blasted Standard for a further 10 points. The Blasted Standard can be used once per game and grants an extra 2D6 s4 AP- hits when the unit uses a witchfire power. This could be pretty useful in certain circumstances but I guess it all depends on the points. In a larger game where there are points to spare then this could be pretty nifty but in a small game where every point counts it might not be the best investment but that's personal choice really. The other standard unit upgrade is the unit leader which in this case is the Iridescent Horror who is basically a Pink Horror with an extra attack and is a Character. As a character the Iridescent Horror can take up to 20 points of Daemonic Rewards and can get the Etherblade, Greater Etherblade, (for both of those see the Bloodletter entry), a Staff of Change (10 points, s+2, AP4, Concussive, causes any enemy character or monstrous creature that is killed by the staff to explode causing D6 s5 AP- hits to all units in a D6" range) or a Mutating Warpblade (20 points, AP3, Specialist, can create a Chaos Spawn on a 2+ if character or Monstrous Creature is killed by weapon).
Upgrading the Iridescent Horror in my opinion is much like upgrading the other Troop unit champions; pretty pointless really and especially in this case unless you're going to try to get the defensive upgrades but even then I think the points could be better spent elsewhere. Going for the close combat weapons for the Iridescent Horror is incredibly pointless because you really don't want any of your Horrors in close combat at all. I repeat; no Horrors in close combat. If they get into combat then they're not too shabby but their role is ranged attack and ranged attack only.
A unit of Horrors uses Psychic powers for its shooting attacks and draws powers from the Change discipline. These powers have been slated by the online community but I think that the main issue is that they don't have any additional effects and they're purely damage based but that doesn't necessarily make them bad. In fact I think they're pretty decent powers and each one has a use much like weapon options for other units. Now before we go into the different powers I'll just mention the one rule that they all have in common that has the potential to really screw things up a lot; Warpflame. Warpflame forces units wounded by attacks with this rule to take a toughness test. If the test is failed then the unit suffers D3 wounds with no armour or cover saves allowed but if the unit passes the test then it gains the Feel no Pain (6+) rule. If the unit already has Feel no Pain then it gains +1 to the existing rule so if the unit has FnP (6+) and it passes the toughness test then it will have FnP (5+) which isn't good but it's the risk you take with Tzeentch.
Ok so onto the Primaris power, the Flickering Fire of Tzeentch. This power has a range of 24", s5, AP4, Assault 2D6, Warpflame (see above) and the Soul Blaze special rule. Soul Blaze means yet more counters to mark a unit but this time it marks the enemy unit. that suffered an unsaved wound from an attack with this special rule. Each turn a dice is rolled per counter and on a result of 1-3 the counter is removed and nothing happens but on a roll of 4-6 the unit takes D3 s4 AP5 hits that ignore cover saves. I assume that armour and invulnerable saves can be taken as normal. When combined with the Warpflame rule this can add an extra 2-6 wounds depending on dice rolls. An interesting thing about this power is that allows for more Warp Charges to be expended in order to get an extra D6 for the shots so expending 1 Warp Charge gets you 2D6 attacks, 2 Warp Charges gets you 3D6 and expending 3 Warp Charges gets you 4D6 shots. If you have a large unit of Horrors then it makes sense to be using this many Warp Charges each time to really maximise the damage the unit is doing. Now while you are getting pretty decent hitting potential here (anywhere between 4 and 24 s5 attacks) there are a couple of drawbacks and the first is the AP value. With an AP value of 4 I think this is geared more towards fighting a hoard army than marines with their 3+ armour saves but with a bit of luck the Horrors can take down a Tactical Squad without too much difficulty, you're just not getting as many casualties because they can use their Armour saves. The second drawback is that because the weapon of choice of the Horrors are Psychic powers they can't fire Overwatch so if they get charged then they're getting charged because you have no way to stop the charge and in this situation the Horrors are, more likely than not, going to get hammered. There are a couple of things you can do to cushion the blow but we'll get to those a little later. Also I should mention that no matter how many Warp Charges you use or don't use you can still only use one Witchfire power so you can't just use one Charge, then use another straight after for a different attack in the same turn.
The next power in the Change discipline is Tzeentch's Firestorm which has a 24" range, sD6+1, Assault 1, Blast Witchfire power that costs 1 Warp Charge. This power automatically hits, pending the scatterdice, and uses the small blast template. This is again useful against a hoard army but if there's a decent roll on the strength dice then it could also be an effective anti-armour attack but with a lack of an AP value this is probably best left to attacking units that don't have a very high toughness value. The next power is the Bolt of Change that again has a range of 24" (a lot of that going around this discipline), sD6+4, AP 2, Assault 1, Warpfire, using 1 Warp Charge and is a beam which means that it will hit every unit that falls in the line of fire. What you get here is a long range, multiple hitting shooting attack with a minimum strength of 5 and a maximum strength of 10. The problem with this attack is that it hits ALL units within the forward range so if the line falls across friendly units then they take a hit. If they're one of the last units hit then it's not too much of a problem because of the fact that each hit reduces the strength of the attack (first hit is s10, second at s9 etc.) but if they're first or second in the line of fire then that unit is pretty much dead. The last power in the Change discipline is Infernal Gateway which is the only power to not have a 24" range but it does have a range of 18", sD6+4, AP 1, Assault 1, Blast, Warpflame that takes 2 Warp Charges. This power would be great against a unit of Terminators that are all bunched together so long as the blast template doesn't scatter. This would also be excellent against vehicles so this really would be an anti-armour power because each hit against a vehicle would get at least a Crew Stunned result and it becomes much easier to destroy a vehicle. Plus it will literally kick ass but the downside is that it is the small template and the BS of the Horrors is 3 so the blast marker is likely to scatter a decent amount. While this isn't exactly a problem this power is only really available for use by a Horror squad of 11-15 models because of the 2 Warp Charge cost and it's not really a good use of Warp Charges for a 16-20 model unit because they get 3 Charges so you're actually wasting a whole charge.
The Horrors most certainly are a purely shooty unit and are best kept so far out of close combat that they're basically guessing at where the targets are because barring some very lucky dice rolls they are going to get slaughtered pretty quickly. Now unlike most other shooting units there's very little room for customisation because you only get one "weapon" and once it gets rolled for you're pretty much stuck with it. This isn't to say that the Horrors aren't useful they're just of limited use. They also have a chance to have their entire ability to cause damage shut down by a successful Deny the Witch roll which would obviously be bad. Very very bad because they're one of only a few shooting units in the army. Now earlier I mentioned that there are a few things that a Daemon player can do to protect the Horrors and one of them involves cover and Going to Ground so that they get a kick ass Cover save (preferably a 2+) which combines with the Daemon of Tzeentch rule that allows for re-rolls of saving throw results of 1 which practically makes them immortal barring Ignores Cover attacks. The reason this works is that the rules don't say anything about no Psychic powers when going to ground so the +1 to the cover save adds to their survivability.
As with all of the units in the Daemon Codex the Horrors fill a specific role and in this case it's a basic ranged attack. Unless there's a successful Deny the Witch roll there's a good chance that they can do a decent amount of damage to anything not wearing Power Armour. My advice with units of Horrors is to choose their powers based on what you're running against; a lot of armour needs high strength and AP but lots of infantry needs a lot of hits and this flexibility is actually an advantage of the random nature of the Daemons because tactics can be adapted on the fly and your opponent can't accuse you of cheating. The main rule is to keep them far from combat, deep in cover and let the fire fly. Use them to start softening up the enemy so that they're ready to be obliterated by the close combat focused Daemons with the various psychic powers and you're good to go.