The Ceremonial Trident is essentially a counterfeit Lightscale Trident, but can be reforged by Dento in Zora's Domain, which is also where you'll find it.
Zora's Domain is to the east of Central Hyrule.
From Mipha's statue, head up the staircase to the left.
Make your way to the bridge leading to the west gate of Zora's Domain.
You need to jump off the right side of the bridge and glide down to the very bottom level of the outer ring.
You could swim there, but it wouldn't be as quick.
Get yourself to the spot on the map above.
Facing away from Zora's Domain, use Magnesis to spot the Trident in the water.
Use ice blocks to reach it and switch to Magnesis to lift it out and onto solid ground.
If you break or lose the trident, bring Dento a Zora Spear and five pieces of flint, and he'll make you a shiny new one.