Sea monster or not, we're not going to risk the safety of our shipping. I've ordered our best marksmen to prepare for a confrontation with this Tethyr. The artillerymen have augmented the dock's defenses with three mortars.
I'm going to order the lighthouse's lamp illuminated. If Tethyr does attack, it will be your job to operate the mortars and take down the beast before it can kill all of the marksmen. I wouldn't stand in one place too long, though. Using the mortars is likely to draw it's attention.
Well done, <name>! I'll admit, when I caught my first glimpse of that beast, I had doubts about our ability to defend the docks, but you held together the defenders brilliantly!
Use the mortars on the docks nearest Tethyr to speed the process and successfully complete the quest. It is not necessary to tag Tethyr to complete the quest.
When the quest starts, it automatically begins to rain and will not stop until the encounter is over.
Falling in the water or going too far away from the docks will fail you and you'll have to restart the quest. On rare occasions, if you restart a failed quest too quickly (while Tethyr is still alive and leaving the dock area), he will not come forwards for the new quest, and you'll have to intentionally fail and restart.
This author observed that a previous fail on the part of another player which involved the deaths of all the marksmen perhaps also caused the death of Major Mills. After approximately 5 minutes, Major Mills respawned and this author was able to start the quest. If Major Mills cannot be found, it's possible a previous player catastrophically failed moments ago; be patient, he'll show up where he should be.