Pink Horrors are a bit of a weird one in the Chaos Daemons codex thanks to the weird Tzeentch power rules. However they are the only Chaos Daemon troop choice which offers ranged fire support and despite having the weird warpflame rule, they do have a place in a balanced Chaos Daemons army.
All about the pink, baby
The Pink Horror costs a low 9 points and has a profile the same as a generic Guardsman, so nothing to write home about.
No surprises they have the Daemons of Tzeentch special rule; this allows them to re-roll 1's and gives them +3 when casting psychic powers, handy as their leadership is 7. They also have Hated (Daemons of Nurgle).
Horrors have a little gimmick rule for creating Blue Horrors. When a Pink Horror is slayed in combat place a counter, for each counter an enemy unit engaged in close combat receives S2 AP2 hit at I1. Not awesome, but it's a funky way of producing Blue Horrors for the fluff bunnies.
Psychic power wise the Horrors have the Brotherhood of Sorcerers rule and also if numbering 11-15 they generate 2 warp charge points and if numbering 16-20 they generate 3 warp charge points. Don''t forget that the Brotherhood of Sorcerers rule makes the Pink Horrors mastery level 1, so they will be denying the witch on a 5+.
The Force is strong with this one
Pink Horrors have access to powers on the change discipline. The standard power is Flickering Fires of Tzeentch which is S5 AP4 24" range, also has the soulblaze rule by the way. More interestingly this is assault 2D6 and for each warp charge point expended the Horrors can fire an extra D6 shots - so a unit of 16 minimum Horrors are going to fire 4D6 shots, that's about 14-16 shots if I am correct! The flickering fire has AP4 which is excellent for totalling xeno forces.
Other powers are Tzeentch's Firestorm which is S6: D6+1 blast; Bolt of Change which is S: D6 +4 AP2 and Infernal Gateway which is S: D6 +4 AP1 blast. The former and latter are both witchfire powers while Bolt of Change is a beam.
It is always worth rolling on the psychic power table as you may get something useful. Personally I do not favour the firestorm power, though bolt could be useful. Unfortunately gateway is useless as the Horrors are only mastery level 1 and can only generate as many warp charge points as mastery level. The flickering fire is the standard power so you can always switch it if you do not like the result, plus at least you know where you stand with this power in terms of strength.
All the powers above have the warpflame rule, basically an enemy unit takes a toughness test at the end of the phase if the unit suffered an unsaved wound. If the test is failed the unit takes D3 wounds with no armour or cover saves allowed. If the unit passes toughness test then it gets feel no pain (6+), if the unit already has feel no pain then it will get +1.
My little Pinkies
You can upgrade one Pink Horror to be a squad leader, who can take lesser and greater rewards. If I am honest I think you're wasting your time doing this - the squad leader offers no benefits.
You can take icons which have the usual none scatter deep strike bonus. You can upgrade the icon to be a blasted standard, this does an extra 2D6 S4 AP- hits when an enemy unit is hit by a shooting attack. You can only use this banner once and for that reason I would say it is not worth it.
Also can take an instrument which lets you bring on another unit automatically (providing your unit passes reserve roll) or re-roll warp storm table results. Personally I would save the points.
Rolling with the Pink Unit Sizes
Pink Horrors can come in maximum unit sizes of 20 and cost a reasonable 180 points. In this form they are excellent for destroying enemy infantry, taking wounds from monstrous creatures and glancing transport vehicles to death.
While the amount of shots is attractive, I personally wouldn't run large Pink Blobs because the biggest issue with Horrors is deny the witch and you do not want all that fire power denied with a single (lucky) roll.
Instead you may want to consider units of 10 or even 13 Pink Horrors. Units of 10 is going the MSU route and will fire on average 7 shots; still handy against small infantry units such as Tactical Squads etc. Units of 13 Horrors will allow you to rock with warp charge 2 and you can fire 3D6 shots instead, you also have those two extra Horrors to absorb a little punishment before your number of flickering fire shots diminishes.
There's a few options how to use Horrors on the table top:
You can keep them sitting on an objective in area terrain, this means not only do they claim the objective for you but also get some kind of a cover save. Now I know they have the Daemon rule (5+ invulnerable and Fear) but if you're inside a ruin that's 4+ cover. Keep in mind that if you go to ground in area terrain you get a +2 to your cover saves. Thanks to the 24" of flickering fire the Pink Horrors can reach out and touch someone.
At this point it is worth mentioning Plaguebearers and I suspect you're thinking about them too. Plaguebearers are more survivable thanks to shrouding and T4, in fact they are the most survivable of the Chaos Daemons troop choice. The reason for not using Plaguebearers as dedicated objective holders (or Grot Squad) is because all they do is standard there while Pink Horrors can at least contribute with ranged fire power. If Pink Horrors go to ground they will get a +2 cover save, which will work out the same as the Plaguebearers.
Another option for Pink Horror deployment is use them in tandem with a fast moving assault unit i.e Flesh Hounds or Seekers. The Pink Horrors can move slightly up the board and then camp in terrain to get a better cover save. The opponent has the choice then of tackling the on coming fast assault units or the long range shooting Pink Horrors. Either one is a difficult choice.
All Daemons have the deep strike option and for a smaller unit of Pink Horrors this is a valid choice; drop them in at the rear of the opponent's deployment zone and the little buggers can pester camping units such as Long Fangs and dedicated objective holding units such as Cultists or Grots. Providing the Pink Horrors survive they can also bag you line breaker. Makes a great cheap distraction unit. You can deep strike them in terrain if you want the better cover save, just remember counts as dangerous though ;)
The biggest survival tactic for Pink Horrors is keeping them out of LOS. Because the way Horrors shoot you only need one of them to get LOS on the enemy unit, then they can do their psychic business. The opponent then will have to find a way to draw LOS on them or fire at the models in sight and only them will be causalities if they fail saves. When Horrors are camping in terrain and only a few can be spotted it makes those Horrors go a long way!
I am not going to lie and say Pink Horrors inflict shooting slaughter, truth is they don't. What they do is deplete enemy units and soften them up prior to Daemonic assault.
If you're using flickering fire then Pink Horrors are great at blasting xenos to bits (AP4) and small infantry size units such as Veteran Guardsmen, Tactical Squads etc. They can also deal with light armour, but you really have to roll well and glance most things to death. Ideally they are there to soften up enemy units prior to assault or just deplete enemy fire power and keep the pressure on.
The other powers are pretty random due to the strength, though firestorm is ok for hordes while bolt and gateway and handy for blasting vehicle car parks.
Just don't expect Horrors to murder units with weight of fire and always expect deny the witch, that's one of Horrors' achilles heal along with the warpflame rule.
Now if you are worried about warpflame, just fire at vehicles instead or deplete the enemy unit so only a few models remain and they are not mega effective after that or can take much punishment even with feel no pain. Just remember it only gives 6+ feel no pain, so not mega awesome, but still a free boost to the opponent.
Herald of Tzeentch
When talking about Pink Horrors the Herald of Tzeentch is always going to come into the mix.
The Herald is a cheap supporting psyker and can really make Pink Horrors shine. The Herald has access to the change and divination disciplines. Switching to divination gives access to prescience (re-roll to hit) very handy considering the Horrors are BS3 and also reasonable for dealing with flyers.
Just keep in mind that the Herald is really only worth it in larger Horror units in order to make the most of all that fire power and also keep in mind that if deny the witch goes off that's a lot of points wasted. I really do not think the Herald is worth it in small Horror units for prescience use. Really is a catch 22 situation; take larger units and get better benefit but deny the witch is a shitter for that amounts of points or take smaller units and do not get a bigger return. I think smaller units is lesser of the two evils.
If you do not want to use the Herald's ability for prescience you could just use flickering fire for adding shooty bonus or anything else from the change discipline. That would be 4D6 shots with a unit of 10 Pink Horrors and a supporting Herald of Tzeentch.
Another use of the Herald is the locus abilities. The most interesting locus is locus of conjuration, this gives +1 strength to to all hits caused by psychic powers. If you're using flickering fires that's S6 (good for insta gibbing Eldar, Dark Eldar, Guard, Tau etc), rear armour of flyers i.e Vendetta, general anti armour and monstrous creature slaying. The other powers on average will be S9; nasty if you get the beam power.
Pink Horrors, how do you runs yours? MSU or large blobs? Do you throw in a Herald for supportive goodness?