The lord of Clan Cadarn.
At the end of Regicide Arianwyn stops the player before they can deliver a Message from Lord Iorwerth to King Lathas and will break the seal on the message and tell you to read it. This reveals that you have been tricked by both men into helping them with their plan to destroy Camelot and Arianwyn will ask you to become his inside man and help the resistance.
During Mourning's Ends Part I, Mourning's Ends Part II and Within the Light you will infiltrate the group of Mourners in control of East Ardougne at Arianwyn's request and learn what they're up to in regards to both the city and the subsequent discovery of the Temple of Light beneath it.
During Plague's End it is revealed that Arianwyn is actually Lord Cadarn and you must help him and the other elven clan lords re-grow the crystal city of Prifddinas to prevent Lord Iorwerth from summoning the Dark Lord.
After the quest he can be found near the Cadarn Herald in Prifddinas in addition to Lletya.