This list focuses on the Slaves to darkness book strengths, trying to squeeze out the maximum of those, mainly focusing on: synergy and the spell lore.
Most importantly, the ritual. I took Mighty ritualist trait for my general, so the ritual is more likely to succeed: d3 models slain from a unit, and you get that many to your casting rolls. You want to sacrifice models from splintered fang, this unit is there to hold your backline objective and be the fuel to the ritual, since you get back one model to this unit a turn.
This ritual is important, as the most potent spells in the lore have high casting values of 7, and also gives a fail safe to the spite tongue curse.
Also worth noting, every wizard knows a spell to manipulate movement value of enemy units by d3, sometimes might come handy.
Picked mark of Tzeentch for the wizards, and undivided for the rest of the army. Tzeentch, so they can get another layer of synergy with the blue scribes, and undivided so the marauder units low bravery wont come into play. Also, chaos warshrines buff will help bring out the best from undivided marauders.
Artefacts, Command Traits & Abilities
Mighty ritualist: adds 1 to your ritual rolls, so you pass it on 2+
Thermalriders cloak will help the chaos lord to keep up with the marauders, so it can use your command points to make them fight twice.
Be’Lakor: casts two spells, decent in combat, and an utterly disgusting ability which can buy you a turn, sometimes blocking half of the opponents army, if caught off guard. He is quite mobile, should around teleported marauders, or harassing something on its own.
Chaos lord: insanely good command ability, allowing something to fight twice, follows the other marauder unit.
Chaos sorcerer lord: re-roll armour save buff is very strong, its warscroll spell is insanely good.
Gaunt summoner: two spells, summons a unit of pink horrors, which is hard to get rid of. Those horrors are also wizards, which will come into play with Darkfire demonrift.
The Blue Scribes: now, here comes the tricky part. On a 4+ lets Tzeentch wizards re-roll casting rolls. With bonuses to casts, and this re-rolls, your spells are likely to go off. And that is very important, with the teleport spell, but more on that later. His other ability to learn spells, and cast them later on on a two up makes him a dickhead. Put it on a Balewind vortex turn one, so you will be able to cast its warscroll spell, and also to read its scrolls. Try to teach it the teleport spell as quickly as possible, even if you do a meaningless teleport, just do it.
Marauder horsemen: your harassers, and they will be the first to arrive to objectives. If you castle up, use them as screens.
Marauders: you know them, you love them. They will be your objective takers, they will kill anything if supported correctly: buff them with either warshrine or sorcerer lord so they re-roll all hits and wounds. They charge minimum 8, so they are very likely to make the charge after being teleported. If you manage the prayer, they fail the charge only on 4 ones, which is quite unlikely to happen. Buff them, teleport them, and enjoy your opponents stress.
Splintered fang: your ritual fuel.
Warshrine: the buffmachine. If someone makes close to your lines, you can throw this into the combat. Try to keep its 6+ fnp aura close to your units.
They are expensive AF, and usually not worth the points due to unit restrictions, or the useless ability. This list isn’t cp hungry, and you can get away with one artifact. Also, this list is happy to be either player one or two. Deploy your chaff first, so you can see where your opponent sets up, so you can better counter it. But, teleport can fix any deployment mistake you make. If you want to mock your opponent, deploy one of your marauder unit on the table edge, just to let it know you’ll teleport it, and its anywhere on the battlefield, and you will capitalise on any mistakes/gaps it leaves.
The Blue Scribes. Magic fail safe, your source of second teleport on a 2+, if you managed to teach it the spell.
Magic, Endless & Realm Spells
Whispers of chaos: 7+, d3 mw, if a model is slain, they cannot move. Be’Lakor is mobile, use it to get close to a unit, and cripple it. Down to personal preference, you might want to take a different spell for him:
Binding damnation: 7+ ,makes a unit fight last. If you pulled off a double charge with both marauder units, you wont need to chose which one to fight with first. Also, useful against certain heroes.
Mask of darkness: 7+, your teleport, known by gaunt summoner.
Spite-tongue curse: 3+. Some might argue there are better spells in the lore, and they would be right(see Be’Lakors choice). But, with cabalists this spell will almost always go off, and does straight 3 mw to anything. Yes, get rid of those pesky heroes. If you send it through the Umbral spellportal, you don’t need to worry about the opponent dispelling it.
Balewind vortex: Meant for the Blue Scribes, don’t touch it!
Umbral spellportal: set it up turn one. Just do it. Allows you to make your first turn impactful, you have many nasty things to send through the portal, pick which is best at the situation.
Darkfire demonrift: now this is some serious gourmet shit, if used correctly. Put up your endless spells, get as many wizards close as possible, and go into town. If you just send it through the portal turn one, it will already do quite alright amount of damage, but if used correctly, the amount of damage will skyrocket. Tzeentch armies will especially love you if you send them Archaons regards in the form of this endless spell.
I play this list cagey in the first one-two turns. Many people lose their games, because they rush into combat. Patience, my friend, go into those fights you can win for sure, avoid the rest. Preserve your assets, don’t throw them away. With this list, you can be a true Tzeentch general, act like it. In those first turns, soften up your targets, and prepare for your strikes. Your opponent will get ahead of you on victory points, but you know that will happen. Stay calm, and keep casting spells. You will kill all your opponents, you know it, you’ll catch up. (try to fork in as much vp as possible even in these turns) An opponent in lead is more likely to become overconfident, and make mistakes. Force those mistakes, get into your opponents head with your tricks. And, while your opponent will stretch out its lines… your marauders will have more place to be teleported. Once the gap is there, pounce, cut throats and claim the objectives. Kill, maim, burn! Ah,no, that is a different army. Anyway, you know what I mean. Do it. You will be in control of the game. Use it to let your opponent have fun too with a close game, try to take lead only in turn 4 or 5, so both of you will have a positive game experience. Close games are the best, try not to stomp. I know you could, but just don’t. You don’t need to show all your powers, Tzeentch knows.