In order to discover Cromlin island, you need to talk to Traabi the healer in the Elven Caravan. Chat with him, he will complain about his problems... It seems that you can help him if ever you can find a very rare flower, the Blue Lotus. Unfortunately, he doesn't know where to find it, but maybe the farmers of the Tyrian Fields can have some clues. Anyway, be courageous, say that you'll find the Lotus for him!

Go to the Tyrian Fields and talk to Pa, who is much more knowledgeable than she looks at first sight. She will mention that the blue Lotus might be found in a certain island, near the vicinity of Cromlin.

Go to Altaruk, and talk to Larak, you'll notice that you can now access to a island near Cromlin...