The Sun Stone can be found halfway between Ashfallow Citadel and Tel Mitryn. Head there and use the Bend Will Shout on the obelisk in the middle. That way you will summon a Lurker and make the local Bandits (who have been busy with building up until now) to run away. You will have to fight the monster on your own - now one will help you, unless by accident.
After dealing with the threat, touch the cleansed obelisk to learn the Sun Flare. It's a quite powerful magic skill which will help you fight big groups of enemies.