Seggin wants to see the sight of Princess Mipha holding the Lightscale Trident in the light of the moon.
Quest Location
Talk to Seggin in Zora's Domain to begin the quest.
How to Complete the Quest
On a clear night, go to Ihen-a shrine (east of Zora's Domain) and go south up the stairs to the statue of Mipha. If the moon hasn't risen yet, you will need to wait for it to rise at around 10PM in the game. Then you can take a picture of the statue of Mipha from any angle, and if the picture has a red exclamation mark, you can go back and talk to Seggin to complete the quest.
Quest Reward
Seggin gives you a Zora Sword.