Herud in the castle will tell you about Evenorn and Llarien will tell you more if he's in your party.
In the mines you'll find a book on the body of Florien which will tell you about Evenorn IF you have been to the castle first. If not, the quest will be a dead end and if you haven't talked to Herud at the castle you will not know enough about Evenorn to continue your way south. You'll be stuck.
The only one who can help you now is Llarien. You have to have rescued him in the mines and he has to be part of your party. He'll then tell you about Evenorn when you try to go south in the forest and you'll be able to get to the city of elves. Without Llarien you won't be able to get there.