Stealing anothers soul is possible, with effort, especially when they are asleep or otherwise knocked out. Taking it when they are awake it a lot harder.
And yes, damage is permanent. Although if you are just messing around it isn't likely that you will destroy the entire thing in one go, but summoning things tends to take its toll heavily.
As for inscribing on weapons, maybe not in the first iteration, but most likely one day. It would be a simple case of adding glyphs for ON_WEAPON_HIT and such.
Right now I'm taking a break from programming to do some drawing. While cultist is a roguelike, it will have its own symbol set. Included so far are 16 glyphs, and soon to be 16 marks.
While currently there are only 16 visually distinct glyphs, there will be more than that functionally, so while they will be useful for reading spells once you have a few you like to use, names will still be important for telling them apart.
Marks will be used as religious symbols and tribal tattoos. There will be some times when you walk into a village and notice everybody has the same mark tattooed on their back or face or something of that nature. Nothing to worry about, tribal tattoos are some what common! It is if you then find a bloody alter stacked with human skulls with the same mark that you might start to worry that the locals aren't as friendly as they seem. Gods and demons will like to adopt marks and colors to identify themselves and their property. If you find yourself worshiping a demon, don't be surprised if it brands you with a huge sigil across your chest. That might be the closest thing it shows you to kindness.
Edit:
Ok, on the left 16 glyphs, on the right 16 marks.
Edit: Wait, did a bad job with the copy and paste and missed the bottom two! Ah well, they can be a mystery. 15 of each, but 2 more exist.