Tethyr

Tethyr is a level 40 elite (boss) who is part of the questline.

Background
Tethyr used to attack Theramore whenever the lighthouse was turned on, forcing the people of Theramore to only turn it on for short periods of time, but the attacks became too frequent and the lighthouse was turned off "for good". Years later however, Tethyr hadn't attacked as the lighthouse wasn't turned on, so the people of Theramore wanted to see what would happen if the lighthouse was turned on. Tethyr attacked again, but cannons had been stationed and were used to finally slay the Kraken after all of those years.

Tactics
Use the three mortars on the docks to do massive amounts of damage to Tethyr. Using the mortars will cause Tethyr to Spout along that side of the dock. The Spout causes a knockback but otherwise does no damage. Getting knocked too far away from the dock will cause the quest to fail. Kill Tethyr before he kills all of the bowmen on the docks.