King Lathas is the king of the eastern part of Ardougne. He is the brother of King Tyras from the western part of Ardougne. His father was King Ulthas. He plays a part in the Biohazard, Regicide, and Underground Pass quests, along with a couple of others. He had his brother, King Tyras, assassinated by the player. He wants to use the evil elves as warriors to take over Camelot, but at the same time the elves are using him to find the Temple of Light. When you first start the quest series you are led to believe that the king is good. Later on in the quest series you find out that he is, in fact, evil.
This chest is located in King Lathas' castle. Go up the stairs in the castle that is located on the West side of Ardougne East. Then use a lockpick on the North or South door. Walk around until you see the chest. Check it for traps first and then open it.
You can temporarily boost your thieving with these: