What is Deep Blue Sorrow in NTE? How to complete it?

Deep Blue Sorrow is an important world exploration event in NTE related to the deep sea/underwater areas.

Trigger Conditions:

1) Complete Main Story Chapter 4 to unlock the sea area;

2) Find the "Deep Blue Crystal" item while exploring the deep sea;

3) Go to the designated underwater cave entrance to trigger the story.

Walkthrough:

1) Enter the underwater cave and collect 3 Deep Blue Fragments along the route;

2) Solve the water pattern mechanism puzzle to open the passage;

3) Defeat the guardian boss "Abyss Watcher" (recommended level 35+, Ice characters effective);

4) Complete the final story dialogue for rewards.

Rewards include: Fons x200, rare Disc blueprint, Deep Blue title, and achievement.