This is a challenging quest (mainly due to the locations of the Mobs).
Here is some information:
Gwydion is a renowned scoundrel who roams far and wide seeking valuables. The problem is . . they belong to other people.
He has amassed a large collection of Artifacts along with other special items. Sometimes he has these in his pack if you slay him. The Elven Windsong bow has a chance to drop as a reward along with Archery Powerscrolls and many other things.
Be Warned: he is not an easy foe. He has his tricks and he is usually in dangerous Tartarus locations.
To find Gwydion, talk to the elf Dokkalfr son of Freyr. He can be found in the 'Revenge Shop' in the southeast part of Papua in Trammel.
If you accept this quest you have 5 hours to locate Gwydion.