"That Which Sleeps is a turn based strategy game in which the player takes on the role of an awakened evil from the world’s past and must manipulate a living, reactive world from the shadows using a cast of unique corrupted Agents. The player must balance an aggressive approach utilizing his considerable power and eager hordes with the need to remain unknown to the Champions and Heroes of the world.
Corrupt, conquer, charm, and manipulate - with a fully realized world driven by thematic AIs that follow their own interests you can pursue multiple paths to victory.
Customize the world with your choices at the beginning of each scenario, choosing an Ancient Evil and selecting his origins and what dire deeds he has wrought on the world.
Make difficult choices with limited agents, choosing to send your Prophet to rouse the orc hordes or beset Kings with maddening dreams will greatly change the flow of the game.
Disrupt the Balance of Power by creating famines in the farmland, stoking the flame of old vendettas, or recruiting pirates to the sea.
Watch the fame of your Agents rise and heroes will begin to hunt them down. Let them fall as scapegoats and keep your name secret, or imbue them with dark power and take the fight back to the heroes.
Each action you take leaves clues behind that cunning heroes can decipher, eventually leading them to the truth. Kill or corrupt them before they find a receptive ear at court."
Not an RPG but The Darkness II has the protagonist inhabited by a demonic evil which manifests as black tentacle-like appendages which tear people apart, eat their hearts and other fun stuff as you murder your way through the game.
I’m late to this thread but I wanted to mention Tyranny by Obsidian. It’s set in a world where a Sauron-like figure has prevailed. You play as a Fatebinder - a sort of judge working for the reigning Evil Overlord.
You can read more about the world of Tyranny here -
I won’t swear all of those will let you use “black magic” but I’m pretty sure all have magic and evil playable people/civs.
Also try the Overlord series. From what I remember you are mostly controlling evil minions, but you also have spells.
It’s stretching the definition of “black magic” a long, long way… but Star Wars: Knights of the Old Republic allows you to play as a dark jedi, using the dark side of the force. This has a deep effect on your game experience and how NPCs interact with you. You can do some evil stuff, both in combat and as part of the story.
It’s not what you’re looking for, but it may well scratch the same itch. Also, it’s a damn good game in its own right, and since it’s old, it’s available cheaply on every format known to man.