Retrieve a Fallen Moonstone from the Shadethicket Oracle, then bring it to Pelturas in Astranaar.
Relara's fever has passed, but she is still very weak. I do not believe time alone will give her back the strength she has lost. I believe it will take the power of a moonstone.
We used to gather moonstones at Fallen Sky Lake to the southeast... until the Shadethickets came.
Their leader, a Shadethicket Oracle, gathered up all the fallen moonstones and hoards them. Now, you must retrieve a moonstone from the Oracle.
And to do that, I fear that you must slay him.