Ask Ossian what you need to do to restore the strength of the Lady of the Lake.

Recover the Ring of Goddess: Waterbinder and Necklace of Goddess: Evergreen. The ring can be obtained from a Pilgrim of Splendor and the necklace can be obtained from a Judge of Splendor.

Monsters to Hunt - Pilgrims of Splendor and Judges of Splendor

You've recovered the Ring of Goddess: Waterbinder and the Necklace of Goddess: Evergreen. Take them to Ossian in the Town of Goddard.

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

Take Rain Song, the Staff of the Goddess, to Ossian in the Town of Goddard.

Deliver the three sacred relics of the goddess to Magister Ladd at the Ivory Tower. He's discovered a way to tap the sacred power contained within them.