I think I’m a bigger fan of magic being intrinsic to the world/humans.

You might get a kick out of the Inheritance cycle, it has my favourite magic system to date. Magic comes in the form of knowledge about the “true language” (there called Ancient Language) of the world, as using it means exerting complete control over whatever you named. I quite enjoy it.

It’s a young adult serie so expect some simpler themes, but still quite good imo

I don’t like the idea of mages just being drug addicts, or that it’s similar to drugs.

Differences in opinion, I suppose. Drugs, alcohol and other mind-altering substances are intrinsic to the human experience; but not something required by everyone at all! It’s like music, not everyone cares about it but it’s something that has shaped and been a part of humanity since likely before the inception of the species we are. A good chunk of animals experiment with these. Terence McKenna’s “Stoned Ape Theory” is complete bullshit but it makes good science fiction like most grand, ridiculous conspiracy theories. You can do far more than just street drugs or “drug addicts”. You can make an analogy of oil or natural resources guarded heavily by an elite who seeks to dominate the working class who produces this miraculous substance capable of temporary psychic abilities. I think of Oddworld, perhaps.

[reading Slannesh’s backstory made me realize I was reading something made by the British in the 1900s]

The more modern takes on Slaanesh made me appreciate her more. The chaos gods are, in my opinion, focused on showing the more grotesque, horrible or atrocious acts they commit. They make good stories, indeed. However, Slaanesh isn’t just giant sex demons. It is music, art and creation. It is to entertain and be entertained. Every time you create, make music or make art; you feed her. It isn’t just orgies and space drugs.

with every race having a different reaction (or none at all) to it.

Fabius Bile reacting to Slaanesh is always a classic. I will post my favorite excerpt here!

‘What have you done?’ he demanded. His words fell flat, the echo stifled at conception. ‘Some trick of witchery?’

‘Nothing so crude. Merely a moment, stretched to its utmost.’ The Quaestor floated closer. ‘To my perceptions, all time is thus. A collection of eternal moments, one bleeding into the next with infinite slowness.’

‘Why?’

‘This is the moment of testing. The moment your hearts are weighed against the Phoenix’s feather. Are you not curious at the outcome?’

‘Not remotely. I know my worth, and I know my crimes. This court holds no jurisdiction over me.’ Fabius straightened, trying to slow his heart rate. His muscles strained against unknown pressures. It was as if he stood at the bottom of a vast ocean, and the weight of thousands of fathoms pressed down on him.

‘Its jurisdiction extends far beyond your ability to conceive, alchemist. You have committed crimes of such monstrous elegance that even the gods themselves grow uneasy. Look – see – they sit in judgement of you.’ A too-long finger drifted upwards, and Fabius followed the gesture. He looked up, and something looked down.

It was not a face, for a face was a thing of limits and angles, and what he saw had neither. It stretched as far as his eyes could see, as if it were one with the whole of the sky and the firmament above. Things that might have been eyes, or distant moons or vast constellations of stars, looked down at him, and a gash in the atmosphere twisted like a lover’s smile. It studied him from an impossible distance, and he felt the sharp edge of its gaze cut through him, layer by layer. There was pain, in that gaze, and pleasure as well. Agony and ecstasy, inextricable and inseparable.

With great effort, he tore his gaze away. ‘There is nothing there,’ he snarled, his teeth cracking against each other. His hearts stuttered, suddenly losing their rhythm. He pounded at his chest, as internal defibrillators sent a charge of electricity shrieking through him. The chirurgeon flooded his system with tranquillisers, and he tapped shakily at his vambrace. A secondary solution of mild stimulants joined the tranquillisers, stabilising him. He ignored the urge to look up. There was nothing there. Nothing at all. ‘There is nothing there,’ he said again, tasting blood. ‘There are no gods. Only cold stars and the void.’

The pressure increased. Something whispered, deep within him. It scratched at the walls of his mind, trying to catch his attention. He ignored it. ‘No gods,’ he repeated. ‘Random confluence of celestial phenomena. Interdimensional disasters, echoing outwards through our perceptions. I think, therefore I am. They do not, so they are not.’ He met the Quaestor’s bland gaze unflinchingly. ‘Gods are for the weak. I am not weak.’

The Quaestor nodded expectantly. ‘No.’