Arianwyn is the leader of the rebel elves settlement of Lletya. He is first encountered by players in Ardougne at the end of the quest Regicide, after which he can be interacted with in the building at the very east end of Lletya. Players must have started Mourning's Ends Part I to enter Lletya and encounter Arianwyn. Arianwyn is heavily featured in the quests Mourning's End Part 1 and Mourning's Ends Part II.
'Arianwyn Cadarn' appropriately means 'strong leader' in Welsh.
If players attempt to talk to him after completing the quests they'll receive the "Nothing interesting happens." message.