Ask Ossian what you need to do to restore the strength of the Lady of the Lake.
Go to the Valley of Saints and confront Barakiel, the Flame of Splendor. Slay him and recover the Rain Song, Staff of the Goddess.
Or You need to collect 3 Staff of Goddess: Rain Song Fragment. You can collect these shards from the chest after killing Barakiel. Or, the alternative is that you can obtain 100 Rain Song, Staff of the Goddess Fragments from Wailing of Splendor, Fang of Splendor, Crown of Splendor, and Signet of Splendor.
CAN ONLY BE PASSED BY A SUBCLASS