If you make a Sylvari you fight the shadow of the Dragon in the dream the pale tree tells you it means your ment to fight and kill an elder dragon. The dragon in the dream is there because the nightmare are atacking the dream so why is it a dragon? Because the Sylvari are aculy connected to Mordremoth much like the branded and risen are to the dragons that made them. the Sylvari who have acted in ways that helped the dragon were not under the protection of the pale tree and so Mordremoth could very easily control them. the nightmare is most likely a result of Mordremoth corruption. And as such the wild hunt of all player made Sylvari is not to kill Zhaitan but to kill the true enemy of the pale tree Mordremoth