Fiend mutagen is an alchemy ingredient and is used to craft Fiend decoctions. They act like lesser green mutagens when placed in the diamond-shaped slots in the character panel.
- In an undocumented change for Patch 1.08, all mutagen icons were changed. However, there was still a bug for those on Steam and Origin that didn't reflect this and was later fixed in Patch 1.22. Despite this, some may still see the original icons instead, even with a fully patched game.
- The earliest way to get this mutagen is to do the Return to Crookback Bog quest. The final enemy encountered is a Fiend. All other Fiends encountered in the game have much higher levels.
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