When you say "Carlin" he will react like this: "Nemal: Aaaahh, Carlin, yes. I know this town. Strange graveyard there, isn't it?" He gives a silent hint where you can get his Red Rose for his potion.
The route to find him:
Go down in the old ruin in the desert, go down 5 levels. Walk north around the corner and south, watch out for the Beholder in a cave to the left. Walk south till you see some sand and a ladder up. Go up and go north past a 4-way, till you see a sign and a locked door. Now go up through the ladder left of this. You found Nemal (in the big round cave).
The book containing the recipe for the potion of regained vision is in one of the bookcases in desert dungeon library. To receive a key from Nemal you must have all ingredients listed in the recipe (a Roll, Flour, Dough, a Red Rose, and a Cherry), which aren't too hard to obtain.
Nemal: Hello. How are you? Maybe you can help me: Do my shoes have the same colour?
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?
Nemal: So, did you bring the ingredients with you, stranger?
Nemal: You seem to have them with you. Can you tell me, how many minutes I have to cook them?
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!