By Royal Decree II

Objective
Slay the Frostwyrm that dwells at the Watergod's Altar.

Details
"In the name of the mighty conqueror, King Theodore the Bold, you are hereby ordered to offer up the corpse of a dragon of ice in tribute."

Walkthrough
This quest is only available during Post-Game, acquired from the Arisen's Refuge on Bitterblack Island. The Wyrm located in Watergod's Altar is replaced by a Frostwyrm. It can be found in the same spot where you fight Cyclops during the quest The Watergod's Altar.