What does Tzeentchian sorcery even look like, or is that something we're going to have to find out the had way?
Here, the Lore of Tzeentch, organized within the paradigm of the quest's Relatively Simple / Moderately Complicated / Fiendishly Complex / Battle Magic:
Relatively Simple - Magic 2 required to learn, Magic 3 to cast reliably:
Enrage Beast: Your eyes flash with unwholesome light as you inspire fear in a single ordinary animal nearby. They will act in random ways, such as fleeing, fainting, attacking anyone, or dying.
Flames of Fate: Pale orange flames spring up from the ground around you, which contain visions of the near future. These visions may aid you within minutes.
Moderately Complicated - Magic 3 required to learn, Magic 5 to cast reliably:
Mindfire: You cause a lance of magenta flame to leap from your hand and strike a nearby target, which tears away at the victim's sanity. If the victim's sanity is already in jeopardy, they take physical damage that ignores toughness and armor, but their sanity is somewhat restored. If their sanity would be fully restored from this effect, they lose any insanities on them.
Pink Fire of Tzeentch: With guttering pink flames, you strike a single creature nearby, causing good damage that ignores armor.
Slave to Chaos: You put your body in a deep coma and send your consciousness into another living creature nearby, attacking them psychically and possibly allowing you to control their actions. The possessed subject runs the risk of gaining a mutation at the end of the spell.
Subvert Strength: Your hand glows with queer blue spots. The next nonmagical inanimate object you touch loses its hardness, becoming soft and malleable. Steel bends, glass folds, rock becomes putty. Cast on a weapon, it bends or breaks in combat. On armor it ceases it removes its protection. The object remains soft and wax-like for several minutes.
Transformation of Tzeentch: You attempt to transform an individual nearby and force them to undergo a series of horrific transformations, granting them several mutations. Whilst under this effect, the target is considered helpless.
Tzeentch's Blessing: You cry out to Tzeentch for his blessing, either granting you a mutation or more magical might for a day.
Fiendishly Complex - Magic 5 required to learn, Magic 7 to cast reliably:
Destroy Magic: You draw in the energy found within a targeted magic item, making it lose all magical properties for several minutes. If you cannot contain the energies you gain a mutation.
Dispel Mortal: With a tremendous act of will you attempt to suck a nearby creature into the Realm of Chaos to become a plaything of Daemons.
Tzeentch's Fire Storm: You cause a blasphemous storm of purple flame to erupt anywhere you designate nearby. Those in the flame take great damage, and those slain by it are incinerated, transformed into a pink ash that blows away in the wind and then coalesces into Pink Horrors of Tzeentch, one for each victim slain. They are not under your control, and attack those closest to them. They return to the Realm of Chaos shortly after.
Battle Magics: Spells that can change the face of an entire battlefield. The consequences of miscasting these spells are almost as horrifying as the damage they cause when cast correctly. Magic 7 required to learn, can never be cast reliably.
*Warpflame: All of these spells force those who suffer wounds from them to either take more wounds or briefly gain a natural power of regenerating wounds (or increase such preexisting capacities). Chaos is fickle.
Boon of Magic (Lore Attribute): When you successfully cast a spell from this Lore, you have more chance of increasing your capacity to cast spells.
Blue Fire of Tzeentch (Signature Spell): You consume the bodies of a target unit with coruscating blue flames. Warpflame.
Treason of Tzeentch: You reach your thoughts into the minds of a target unit, tormenting them with subtle whispers that stoke the fires of mistrust and treachery. The affected unit is more vulnerable to fear and less capable of leading others.
Pink Fire of Tzeentch: You envelop a target unit in a cone of magical flame that flows from your hand and damages all who touch it. Warpflame.
Bolt of Change: You hurl a single devastating bolt of energy that blasts through a target unit, wracking their bodies with sickening and uncontrollable mutations. This deals great damage not subject to armor. Warpflame.
Glean Magic: You contest your magical prowess against another Wizard. If you succeed, you steal sorceries from your adversary's mind, damaging them and making them forget one spell, allowing you to cast it like any of your own. Warpflame.
Tzeentch's Firestorm: You launch a searing ball of scarlet flames towards the target unit, engulfing them with balefire. Warpflame.
Infernal Gateway: You open a portal to the dread Realm of Chaos, a magical tear in the mortal plane. The target unit suffers tremendous damage with Warpflame.
As you can see, there's not much in the way of long-lasting effects, as befits the Changer of Ways. So if anyone is thinking of finding some Tzeentchian magic item with an always-on effect to take advantage of... it's not very likely to ever happen. Stasis is not Tzeentch's wheelhouse.