To get Tusk of Monoceros Caeli, you need to complete Enter the Golden House, a Trounce Domain in Liyue. Enter the Golden House is located south of Liyue Harbor.
Teleport to Sea of Clouds, the Teleport Waypoint located at the southernmost part of Liyue Harbor. You can then head straight south, which involves a lot of climbing, or you can follow the path leading from the bridge to get to the Trounce Domain.
Check the image below for the exact location.
Tusk of Monoceros Caeli is rewarded for completing Memories: The Golden Shadow II. To enter Memories: The Golden Shadow II, you need to be Adventure Rank 30, and it is recommended to have your party about Level 70.
Upon Childe'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 Childe 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.)
- Type: Talent Level-Up Material
The following characters use Tusk of Monoceros Caeli for their Talent Leveling
Each Weekly Boss has three Talent Level Up Materials they drop. These are the other two for Childe.