toxicTom: All the truly dark abilities and spells and options (like a pact with an arch-demon) are designed for NPCs and are either useless, impractical or outright dangerous for the standard player character. Of course, in a PnP group, there always is a way ;-) but you have to bend the rules/system/world a bit to make it feasible.
Our group once decided to try out an evil party. We looked into summoning demons, blood magic, had adventures where we searched for everything related to Borbarad¹, were looking into the Nameless²... and... yeah, the rules aren't really made for stuff like that. We all ended up being physically broken lunatics, controlled by various demons! At one point we had to ditch the rules completely. You can't summon a demon for a simple fight in a tavern (especially not when you're just some minion to them *lol*), but if you don't, you're done after two rounds (because of the permanent HP cost of most of our spells and former experiments).
We started as a group of mages who were interested in forbidden books and magic. We ended up as evil creatures you don't speak about. The Draconites³ of Hesinde's⁴ order hunted us and locked up our books and studies in some secret place. I think our characters are still hiding somewhere in the "Schwarzen Lande"⁵ (whatever they're called in English... Shadow Lands? Black Lands?). It was fun, but The Dark Eye simply doesn't work like that.
¹A mage who was into demons and blood magic. His spells are forbidden, his books are banned. There was a huge crisis because of him.
²Basically THE Anti-God. Don't talk about him. Don't read stuff about him. Don't even think about him!
³an order of holy Knights
⁴the God of wisdom and magic
⁵a pretty messed up place, because of what Borbarad did