The only person who can summon the Nymph is the Shaman, who has been killed.
Did you find the Shaman's hut?
The Shaman's hut is the second one east of the story teller's hut.
Read the book you find there to discover how to raise him from the dead.
The Shaman's body is the one you saw earlier which had been drained of blood. A raven should be nearby.