In a Nutshell; The Lore of Tzeentch

Lore Attribute(s);

Fires of Change (Daemons of Chaos):
When a Daemon casts a spell from the Lore of Tzeentch causes one or more unsaved wounds you may choose a unit of Pink Horrors or Screamers within 12" of the caster. Roll a D6 for each wound caused by the spell. on a roll of a 4, 5, or 6, you may add another Pink Screamer. If you roll a 6, you may add another Screamer. Models added in this way are put in the back rank and if the back rank is full then start another rank in the back. Units spawned like this way have no upgrades and confer no additional victory points. If there is no room for the models and/or you don't have the models then they are considered to be lost. This attribute is worthless because Tzeentch is the worst possible Chaos God to run. However, if you like Screamers, then I would suggest using this to people who want to run a lot of chaff in that form.

Boon of Magic (Warriors of Chaos):
For every roll of a 6 on the power dice used to cast the spell, you get another power dice back. Sounds awesome, but the bad thing is that only the Tzeentch caster who cast the spell originally can use the generated power die. Meaning, you can't use a Tzeentch caster to just jerk off and give you more power dice for your other casters to use freely. But if you are running a Level 4 to Tzeentch. then this lore attribute can really help you generate more and more to fuel your magic phases, and more importantly Infernal Gateway. Not bad, but pretty damned nice in comparison to most other lore attributes floating around. 

Special Rule: Warpflame
So pay attention folks, because this is how pretty much why the entire Lore of Tzeentch just collapses upon itself. At the end of each phase any unit that suffered one or more unsaved wounds from an attack with this special rule during the phase must take a Toughness test. If the test is failed, the unit suffers D3 wounds immediately. If the test is passed however then every model in the unit gets a 6+ Regeneration. Any unit targeted again or already has Regeneration adds +1 to their Regen save. Yeah, you know what else sucks? Despite having the word 'flame' in the title, there is no mention of these attacks being FLAMING! Yes, you can literally fuck yourself repeatedly by casting Tzeentch magic on the same target. Knowing this, it kind of makes the entire lore just collapse in on itself, because every spell in the lore save the #2 spell is a Direct Damage or a Magic Missile spell. There's no real way to get away from it, and if you're playing Daemons, there's no way to take a Flaming Banner, which means combats are going to be much harder then usual.

Signature Spell: Blue Fire of Tzeentch
This is a Magic Missile with a rage of 24" and a casting value of 5+ with the Warpflame special rule. It inflicts D6 number of D6 Strength hits. It can be boosted to have a range of 48" and an increased casting value 8+. Again, this thing has two main issues with it. First, it's a Warpflame attack, so basically unless you're hitting humans or elves, you're just going to give whatever you're hitting a Regen save. Second, it's a random D6 Strength of a random D6 number of hits. Unless you are a god of rolling dice, then this spell is going to be worth jack shit. I give it a 2 out of 5, and not necessarily a recommendation to swap for it.

Spell #1: Treason of Tzeentch
A Hex spell with a range of 24" and a casting value of 7+, this spell lasts until the start of the caster's next magic phase. The targeted unit must use the lowest leadership value in the unit, and cannot benefit from the Inspiring Presence and/or the Hold Your Ground! special rules. You can boost the spell to have a range of 48" and a casting value of 10+. This spell is very nice to have in your toolbox. Especially if you're playing Daemons or a Warriors Monster Mash list with lots of Fear and Terror causers in your list. Plus, it can't ever hurt to stop your enemy's units from getting their highest Leadership and a re-roll. I really like this spell because it's got a tactical edge that just really appeals to me. I give it a 3 out of 5, because you will run into Stubborn, Steadfast, and exceptionally high Leadership values where this spell won't really matter at the end of the day. However, people may like this or hate it but it take it for what its worth.

Spell #2: Pink of Fire Tzeentch
A Direct Damage spell with a casting value of 8+ that uses the flame template. You place the template down with it's narrow end touching the front of the caster's base with the large end of the template facing towards the targeted unit. Roll the artillery dice and move the template forwards the number of inches equal to total rolled. If a misfire is rolled, the template remains as it does prior to moving it with the roll of the artillery dice. All models beneath the template suffer a D6 Strength hit with the Warpflame special rule. Yeah, this thing is kind of stupid. While no means as bad as Blue Fire, Pink Fire still suffers from Warpflame useless syndrome. I give it a 2 out of 5. No more, no less.

Spell #3: Bolt of Change
A Magic Missile with a range of 24" and a casting value of 8+. It inflicts a single Strength D6+4 hit with the D3 Multiple Wounds with the Warpflame special rules. This thing penetrates ranks in the same manner as a bolt thrower. No armor saves are allowed against wounds from the Bolt of Change spell. This is really one of the best spells in this lore next to Infernal Gateway. I really like this spell though, and if you are going to be running Tzeentch magic I highly suggest you make a beeline for this spell if you can. I give it a 4 out of 5, and a recommendation to always take it if you play Daemons, that way you can get a little bit more shooting in an army that has almost no ability to clear chaff.

Spell #4: Glean Magic
A Direct Damage spell with a range of 18" and a casting value of 8+. Before I go any further, this spell is fucking beautiful and I wish I had in High Magic. The caster and the target (which must be a Wizard) both roll a D6 and add their Wizard levels. If the target's total is high then the caster's then nothing happens. If your caster's roll is higher then the opponent suffers a Strength 3 hit with the Warpflame special rule, he/she loses a single Wizard level (to a minimum of 0) and forgets one randomly determined spell. The caster then immediately gains the spell and can cast it just like any other spell. Simply swap their lore attribute for the attribute of Lore of Tzeentch. This spell is so damned beautiful I want it so bad! The ability to not only neuter an enemy caster but to also turn his own spell against him is just so awesome! I give it a 5 out of 5, because apart from the Lizardmen, no other army can really do anything like this.

Spell #5: Tzeentch's Firestorm
A Direct Damage spell that uses the small round template and has a casting value of 13+ and a range of 30". Works like every other vortex spell in terms of scattering. All models under the template suffer a Strength D6 Warpflame hit (use the same roll for all models). It can be boosted to use the large template instead and scatter 2D6" for an increased casting value of 16+. You all know my feelings on Flame Storm, this is basically the same as Flame Storm barring it may be higher Strength, isn't Flaming, and may still give Regen to your enemy. I give it a 2 out of 5, because there is that chance to roll really well and just destory. But it's too random for me to really say it's a useful spell. Onto the next one!

Spell #6: Infernal Gateway
This is the final spell in the Lore of Tzeentch, and it is glorious! This is a Direct Damage spell with a range of 24" and a base casting value of 16+. The targeted unit suffers 2D6 numbers of hits resolved at Strength 2D6 with the Warpflame special rule. If an 11 or 12 is rolled for the Strength value, then the unit suffers a 3D6 number of hits rather then 2D6. I like this spell in the same way I like Dwellers Below and Final Transmutation. It's not the random, will of fate, uber-killer that spells like Pit of Shades or Purple Sun. It is still Warpflame, which I think really just makes this lore so damned terrible but the ability to inflict a really high number of hits at a really high Strength make it worth taking. I will give it a 3 out of 5 though because of two issues. The first is still Warpflame, the second is that it's still random in terms of the number of hits and strength that you're going to be hitting at. But it's 2D6 for both the # of hits and the S of said hits, so it's not like you can be rolling  1's and just completely waste this spell. However, it is defininatly one of the more powerful spells in Tzeentch magic so I do recommend to make a run for it if you can.

So that was the Lore of Tzeentch, and to be honest, it's actually pretty fucking awful. There are four spells in this lore that I really like and they are as follows; Treason of Tzeentch, Bolt of Change, Glean Magic, and of course Infernal Gateway. Now what's the problem with that? Well, note that the Signature Spell is not in that list. In my opinion, the way you judge a magical lore as a whole is to look at it's signature spell. Think of it this way. Lore of Fire has Fireball, which tells you the lore uses mostly Direct Damage and Magic Missile spells. Lore of Beasts has Wyssan's Wildform, which tells you that Augments will be the focus of that lore. If you applied that concept to the Lore of Tzeentch, you would come up with the fact that this lore is random, this lore is awkward, and this lore is not worth taking in comparison to others. I will still list optimal uses for both DoC and WoC, as few of them as there are.

Daemons of Chaos: First off, if you're taking this lore, it means you have messed up royally because Daemons of Chaos are a mono-god list apart from their cannons, and if you're taking this lore, then it means you're running an all-Tzeentch list. That in and of itself is a big issue, because Tzeentch daemons are not designed to be in combat, they also have very little mobility and very little protection other their Ward Save. However, if you were just to run a Tzeentch caster on Tzeentch magic, I would highly suggest taking this for magic defense, but only if you manage to get Glean Magic. Daemons don't have access to a Dispel Scroll, so this can kind of make up for it. But if you can't roll that spell, than this strategy just completely collapses in on itself.

Warriors of Chaos: ... Yeah, taking this lore in place of any of the others you have access to doesn't really make all that much sense. Unless you have a hard on for Tzeentch (I know, Tzeentch protects with his bullshit 5+ Parry Save) then I wouldn't really bother taking this lore all that much. Again, if you want to take some more magic defense in the form of Glean Magic, I still don't see too much use for this magical lore.