The book containing the recipe for the potion of regained vision is in one of the bookcases in the library. To receive a key from Nemal you must have all ingredients listed in the recipe.
Player: blind Nemal: Yes, I seem to be blind. But I am not sure - maybe the dungeons are too dark! Player: potion of regained vision
Nemal: I heard of a potion of regained vision ... but I can't remember how to make it! Maybe you can help me. Do you know something about it? Player: yes
Nemal: So, did you bring the ingredients with you, stranger? Player: yes
Nemal: You seem to have them with you. Can you tell me, how many minutes I have to cook them? Player: 31
Nemal: Ah. It seems to work. But what are the words I have to speak? Player: nalus murtu
Nemal: Thank you! NALUS MURTUUU! ... I can see again! To show you, how grateful I am, I'll give you a key. Be wise when using it. I can't tell you, where it matches, but ... take good care, it is useless without mental powers!