The Ymir Flesh can be obtained by purchasing the material from Haldor the merchant, who can be found in the Black Forest. If you have enough gold (120), we recommend you stock up on Ymir Flesh!
It Can Be Spawned Using Spawn Commands
Spawn Command
YmirRemains
The Ymir Flesh can be spawned through the Console Command, provided you don't mind using cheats.
Note that with the release of Patch Notes 0.148.6, when you type "devcommands" into the Console Command, a warning message will appear.
