I've read a lot lately about various tactics for the Daemons and one of the recurring tactics I keep seeing is utilising a Herald of Tzeentch, the Divination specialism and a large unit of Horrors. It involves having a level 3 Herald cast Prescience on a unit of at least 16 horrors (therefore generating 3 Warp Charges thanks to the Magic Made Manifest rule) and then bombarding the enemy with the Flickering Fire of Tzeentch at full power (by expending 3 warp charges the amount of attacks becomes the result of 4D6).
Firstly lets take a look at the points cost of this as a strategy to determine whether it is viable starting with the Herald. The Herald has a base cost of 45 points and comes with Mastery Level 1 and can use the Divination and Change disciplines which is great because that's what we need. Now as we need our Herald to be able to use the Primaris powers of both disciplines he needs to be at least Mastery Level 2 which is an extra 25 points but to really ramp up the sheer weight of attacks he needs to be Level 3 which costs 50 points. A Herald may also take up to 30 points of Daemonic Rewards but I'd probably recommend not taking them to be honest. There's a few of the Greater Rewards that could be quite useful but the chances of getting them just don't justify the points and they're not necessary for this tactic. Another thing that isn't necessary is a Locus upgrade; I had forgotten about these when I tested this out but the Exalted Locus of Conjuration (adds +1 to the strength of Psychic powers) so it would probably be a wise expenditure but as you'll see it's not strictly necessary. The optimum build here is the Herald (45) with a Mastery Level 3 (50) and the Exalted Locus of Conjuration (25) for a total of 120 points. Without the the Locus he costs 95 points and on Mastery Level 2 he costs 70 points but loses a few extra attacks which will become very important later on.
Next to consider are the Pink Horrors which have a base cost of 90 points for a unit of 10. When you see that you have the only ranged troop choice and the potential for some hefty Psychic firepower then 90 points is quite reasonable and the Magic Made Manifest rule justifies extra points invested in the unit size. As I briefly mentioned earlier, this rule allows the unit to generate Warp Charges according to the unit size (basic 10 models generates 1, 11-15 generates 2 and 16-20 generates 3) and it's this rule that gets exploited to maximum effect. By having at least 16 Horrors in the unit it generates 3 warp charges which is useful in conjunction with the Change discipline Primaris power. As a ranged unit that really doesn't fare well in close combat it's pretty much a given that they'll be dug in somewhere that gives them the optimum view of the battlefield so it may be a good idea to give them an Icon as a base for the rest of your army to Deep Strike around. An instrument will help if you bring the Horrors in from reserve but if not then it might keep some units alive if the Storm of Fire result is rolled but these last two aren't strictly necessary. An iridescent Horror is also not a mandatory requirement but the extra attack might help a little should the unit be attacked. Another thing to consider is the Blasted Standard which adds an extra 2D6 attacks to a single Witchfire power per game and while it isn't necessary it is a nice little bonus. For 16 Horrors with no upgrades we're talking 144 points which is a pretty good deal for the level of power we're soon to be discussing but to really give them an edge, the unit should number 20 so that they still generate 3 Warp Charges even after taking a few hits (180), a Blasted Standard (10 for the Icon, 10 for the Blasted Standard so 20) and an Instrument of Chaos (10) for a total of 210. If you really want to then you could add an Iridescent Horror (10) but I would certainly advise against giving it the Daemonic Rewards as they are a waste of points here. For the full upgrades you're talking 210 without and 220 with the Iridescent Horror.
Now we've talked points, lets talk about how the actual strategy works. Firstly the Herald and Horrors chooses their Psychic powers (both choose the Change Primaris power, the Herald takes Prescience from Divination and is then free to choose any other power if it is ML3 which I'll assume it is). Next the Herald manifests Prescience in the Movement phase on the Horrors which gives them re-rolls to hit for the Herald and Horrors as one unit. In the Shooting phase the Horrors and the Herald manifest the Flickering Fires of Tzeentch, expending all available Warp Charges. The Horrors will then get 4D6 attacks and the Herald (in the first turn) will get 3D6 attacks at Strength 5 with AP 4 plus the chance for more wounds via Soul Blaze if the enemy unit survives the onslaught. Now this is where the Locus comes into play as it makes the FFoT Strength 6 instead of 5 so it will wound more often (I'll have to test how often another time). Add that to re-rolled failed To Hit results then you should be firing a large amount of hits in each Shooting phase (in fact I'm pretty sure that it's the most a single unit can fire in the game in one go but I could be wrong).
As I mentioned waaaay back at the beginning of this very long article I actually ran some tests of this strategy by using the profiles in the Daemons Codex and the Space Marine profile from the back of the Rule Book (not the big one, the Dark Vengeance one) and the results were... Well I was surprised let's put it that way. As we all know the Tactical Squads of the Lame Marines have a 3+ armour save which falls just under the AP value of FFoT so I was able to see the average casualty output against a commonly found enemy. For the Herald the lowest amount of hits I rolled on 4 dice was 10 and the highest was 21 (I also did a couple of tests with 24 hits just for comparison but I never actually rolled that high). What I found interesting here was that even though there was a very large variation in the amount of hits there wasn't as much variety in the amount of casualties. The Herald averaged out at almost 4 casualties per round with the Horrors averaging at just over 3. It must be remembered that I had forgotten about the Locus upgrade so these were resolved at Strength 5 so we'd probably be looking at an extra casualty each so 5 and 4. 9 casualties is a good result against an army where the basic squad size is 5. If it wasn't for the And They Shall Know No Fear rule then that final Space Marine would be running back to his Corpse Emperor with his Bolter between his legs but he'd rather stay and die which I'm sure can be accommodated by the rest of the army or he might fall to the Soul Fire or the Warpflame rules.
Against a Hoard army or any unit with an armour save of 4+ this will absolutely tear them apart (I managed to roll a staggering 18 wounds from my Herald alone in one of the tests) unless they have access to cover, a half decent Invulnerable Save or even the Feel No Pain rule. Unfortunately this strategy will struggle against heavily armoured units or armies with a high amount of Psychic resilience (I'm thinking Space Wolves with their bloody Rune Priests off the top of my head). I'm also guessing that with the amount of people discussing the benefits of this strategy (and there are a lot of benefits to it) then the very clued up tacticians out there will have taken this into account and be developing ways around it or ways to combat it; there's only so far that the ability to re-roll saving throws of 1 will go...