Long time lurker here, I made and account just to respond to your post! Don't you feel loved?
I've been playing Mono-Tzeentch armies since Daemons received their own codex, and I have really enjoyed the 6th edition codex. I have a few suggestions, based on my own personal experience with the army.
First, I'd like to discuss Tzeentch Heralds, as in my opinion they are the lynchpin of Mono-Tzeentch armies. Taking the Exalted Loci is a good start, though I would REALLY suggest upgrading them to at least Psyker Mastery level 2. Leaving them at Mastery Level 1 forces you to chose between taking a Tzeentch witchfire spell for offensive use, or taking prescience from the Divination school. Keep in mind Prescience twin-links not just the psyker but also the squad he joins, which is very beneficial to your BS3 Horrors. While on the other hand having the Herald also cast Flickering Fire not only adds a large amount of firepower, but all of his attacks have the chance to be precision shots as well!
My answer to this conundrum is; take both! While expensive in points, the sheer amount in pain these little guys can dish out is amazing. I take Psyker Mastery level 3 Heralds with the Exalted Loci and stick them in squads of 20 Horrors. As said, it makes them expensive but the benefits you gain are worth it in my opinion.
1. You lower the success rate of "Deny the Witch" for most enemy Psykers. Only those rare few with Psyker Mastery Level 4 will deny you on a 4+.
2. You can take both Flickering Fire and Prescience while still getting an extra roll on the Divination table. While only a few spells truly benefit you, the reason I do this is for the chance to roll Misfortune. I cannot fully express how powerful this spell is on these guys. With your potentially
2D6 +
4D6 Twin-Linked strength 6 shots you will cackle manically as you remove Riptides, Dreadknights, and Terminator Assault squads with a single round of shooting.
3. Even if you don't get lucky and roll Misfortune, the extra level will not be wasted! All that changes is now you have an extra point to dump into the Herald's own Flickering Fire attack. Meaning you will have
3D6 shots from him and
4D6 shots from the squad. Once again, all twin linked and strength 6. Even Monstrous Creatures can't scoff at that kind of firepower.
Now, let's look at the Soul Grinder. I'm assuming you took him as an anti-flyer choice. For ANY other Chaos God this would be absolutely necessary, though for Tzeentch our strongest anti-air option is our basic troop choice! (Once again, with the help of Heralds) The Soul Grinder gets 3 shots at BS3 with one point of strength above your Horrors and not being twin linked. While the Soul Grinder isn't a terrible choice, he's just not very effective against flyers in comparison. Be it Storm Ravens or Heldrakes, anything short of a Cestus Assault Craft your horrors can handle.
Last but not least, your Daemon Prince. A powerful and expensive choice. Both him and the Soul Grinder fulfill the roll of assault, should the need arise. Personally however, I wouldn't equip the Grimoire on him. If I was your opponent, I would focus my firepower on whatever
MC was NOT under the Grimoire's effect. Here you'd run into having to chose between making the Fateweaver unkillable with a 2++ rerollable save, or protecting your Daemon Prince. Both are necessary for this trick to work, the Fateweaver allowing you to reroll the Grimoire, and the Daemon Prince holding it. While the Fateweaver's basic 4++ is nice, this is not enough to protect him from most armies. (Especially Tau) Personally, I find the Grimoire a harder target when placed on a Herald of Tzeentch. You have to be careful not to fly out of the Grimoire's 24" range with your
FMCs of course, but considering the Grimoire is used at any point during the movement phase this is actually not as restricting as it might seem. I use the Grimoire to make the Fateweaver himself invincible while the Herald bearing it remains safely tucked away in a squad of 20 Horrors. Remember that you get to reroll a dice courtesy of the Fateweaver each player turn. On your opponents turn, this can be used to reroll grounding tests, or an unlucky failed "Look out Sir" roll for your Heralds!
I will note while the Horrors are the work horse of my army, it's important to note they can't handle everything. Namely
AV 13 and 14 tanks, or simply tanks with long range, can be a real issue. For this reason alone I would suggest looking into taking some Screamers of Tzeentch. They're fast and versatile. Be it assaulting tanks with their armor bane attacks, flying over infantry and attacking them with their tails, or simply assaulting quick units to tie them up and allow your army the time it needs to deal with the threat. I've grown to trust my screamers in a pinch this edition.
Keep in mind Mono Tzeentch is a very strange and easily personalized army. Some people like to play Flying Circus, Others go for Screamer Stars, I personally like a more rounded list with lots of bodies on the board. There's no wrong answer though, what I've said here is simply my personal opinion based on experience with this playstyle.
