To get Dvalin's Sigh, you need to complete Confront Stormterror, a Trounce Domain in Mondstadt. Confront Stormterror is located northwest of Wolvendom.
Teleport to the Domain itself, or the Brightcrown Mountains, the Teleport Waypoint located at the same place.
Check the image below for the exact location.
Dvalin's Sigh is rewarded for completing Memories: Storming Terror IV. To enter Memories: Storming Terror IV, you need to be Adventure Rank 40, and it is recommended to have your party about Level 70.
Upon Dvalin's defeat, a Trounce Blossom will appear, and to claim the rewards, you must spend 30/60 Original Resin. You don't have to claim the rewards if you choose not to do so.
Due to Dvalin being a Weekly Boss, you can only get the rewards once every week. You can defeat the boss as many times as you want; however, you will not get a Trounce Blossom more than once.
The Boss Reward will respawn every Monday at 4 am (Server Time.)
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