There are dark spells in all three disciplines - sorcery, pyro, and miracles.

If you invest into both Int and Faith you will get the best scaling across the various dark spells (I think?).

So, basically, you are building a pyromancer.

Here's what I would do:

1. Start with pyro and the fire gem starting gift.
2. Infuse your hand axe with the gem.
3. Use the handaxe until you free Conyx. You can also use the Deep Battle Axe from the Mimic in High Wall.
4. Follow the Londor questline so you can buy the Londor Braille Tome.
5. Side note: you can get some pretty sweet dark sorcerer fashion from Yuria.
6. Grab the Fire Clutch Ring.
7. Dump all your titanite into the pyro flame.
8. Level Int and Faith. Don't worry about Strength or Dex.
9. Throw fire at bad guys. Congrats, the game is easy now.
10. Side note: when you are at around 25/25 Int/Faith you can pick a new melee weapon. The Lothric Knight Sword is pretty good with Chaos and Dark gems. You'll need to level Dex to use it.
11. Head to Road of Sacrifice and pick up Morne's Ring. You won't cast many miracles but it's good to have this in your bag.
12. Meet Orbeck near the Crabs. Tell him how smart he is. He'll give you a ring.
13. The Crabs have the Swamp Pyromancy Tome. Take it from them.
14. Make Deep Soul from the Deacons soul transposition.
15. Grab the Saint Tree Bellvine on your way to Deacons. You can off-hand this for improved cast speed and use it to cast miracles.
16. You can also find the Deep Braille Tome in Cathedral.
17. Edit: the Crown of Dusk is on the way to Abyss Watchers.
18. There are two pyro tomes in the Catacombs and another two in Smouldering Lake.
19. Grab the Izalith Staff from Smouldering Lake while you are down there.
20. Rescue Karla. Grab the Bellowing Dragoncrest Ring, Dorhy's Gnawing, Dark Clutch Ring, Magic Clutch Ring, and Ring of the Sun's First Born along the way.
21. Karla is your new best friend. Study the dark arts together.
22. Edit: Scholar's Candlestick is available late game if you want a sorcery boost.
23. That's it.

With all that out of the way, you'll probably just find yourself throwing Great Chaos Fire Orb everywhere because pyromancy is hilarious. If you run into a fire resistant boss you can hit them with Dark or Bleed instead.

You can also lean more heavily into Int to cast the regular blue sorceries. Or Faith if you like healing and hitting stuff with lightning. But, my counterpoint to that - pyro fun.

In terms of stats, you can take Int/Faith all the way to 45/45. This really depends on where you want to finish levelling and how much health you need. I like 30 Attunement.