you cannot "choose" the light path. There is no "dark" path in the mod as such, but once you hace conpleted Magnus' quest, you will be given Daark Magis at the Naster level (You still will have to buy the spell though), Since Archibalds door is locked, you cannot learn Dark Nagic at the GM levelm howeveer, There is another guy in the oit who will train you to GM in Blasters, however.Toy will get Nagnus' quest as soon as you return the Souol Jars to Sir Cabegham in Celeste At that point, the pit and all therein become your allies.
If Castle Harmondale respawns (2 years after you first ebtered it) you night lose that stuff, I am not sure on thet though.