Though you can't really create spells, Arx Fatalis has a very unique casting system. You have to use the mouse cursor to draw spell runes on the screen and then you could either cast the spell immediately or save it to cast when you need it. I don't really remember it very well, to be honest. The game itself is fairly old and you might have trouble running it on a newer system.
Bought it off GOG on a whim last night. I'm in a love-hate relationship with it so far. The environment are great and immersive, the character models are ugly as sin however. The magic symbol drawing system is rather fickle and will almost never register my signs no matter how slow and steady I draw. Trying to draw a symbol in the middle of combat is impossible.
I couldn't find where my memorised spells were kept. Closest I could find was a page in my journal that showed the runes, what to do with those runes and in what order to cast a spell is still lost to me.
The UI is rather confusing and it's very mouse driven. Dragging stuff to your inventory on a widescreen monitor isn't all that fun and it took me a while to realise that holding shift will auto move stuff into your inventory, the game kept calling shift "stealth mode" for some reason instead of saying that shift is used for more things than just stealth. Opening/closing UI elements are again confusing and conflicting, sometimes pressing ESC will exit the element and sometimes you have to reclick on the element's icon.
There's no hotkey for inventory for example, at least not one I could find.
The source code was released a while ago and with it came a patch so it worked fine on modern systems. Windows 7 and 1080x1920 works fine for me. I ended up rage quitting the game after I got a fair way in when some kind of uber spider, that looked no different from the others, wrecked my shit. Turns out there's no autosave. Saved three hours ago? Tough.
Never did find a bow, it made my ranged character build rather embarrassing when I had to flail hopelessly at enemies and flee before they got a shot in. Found plenty of arrows though.
Apart from all that I was having fun with it. Melee combat feels alright though I would have praised it much more if there was a way of blocking. Characters and storyline are fairly decent so far, voice acting can be hit and miss.