DISCLAIMER: Your Steps and Tasks may be different
You're asked by Finni to kill Nimh (rat). Upon entering Skona Ravine follow the north wall till you pass Werewolf Skulkers. Between there and the Witch (named NPC human) you will find some Wood Rats that spawn in the early night. If you're lucky one of them will be called Nimh. Plan on camping the spot for a few nights before you find Nimh though. On the plus side there are several yellows and oranges too keep you busy while you wait. (Note: It has been mentioned that the fastest way to get Nimh to spawn is to kill "Brush Cats")
[Step #1] Search for Nimh in Skona Ravine. (loc 20169,5649 also spotted at loc 21749,7300)
I found him at 11am game time.
You receive a Nimh's Curse from Nimh!
You have finished the Nimh's Secret quest.