As it turns out, this shade is the spirit of a pandaren.  Ling is jubilant to be released from his entrapment, and gladly answers Anduin’s request to know what happened.  According to the spirit of the pandaren, long, long ago he brought the Waters here as a gift to the Pearlfin in thanks for something they had done in a still earlier time.  Together, he and the jinyu guarded the spirits from falling into the hands of those who would misuse them- but a darkness seeped from the ground, infecting and destroying those living here.  It continued to wear at them even after they died, trapping them here.

Anduin asks after the nature of the darkness, but Ling is unwilling to explain for fear of the very name of it.  He then indicates the Song of the Vale, and offers the scroll hanging on the wall in thanks, before passing onwards.  Intrigued, Anduin speaks of seeing the Vale of Eternal Blossoms himself, if possible.

With Ling’s shade defeated and everything you came here for done, you can look off to the left for the waterfall that leads back to the mouth of the cave, a convenient exit method.

Heading uphill back to the camp where you found Anduin, you are ready to return to Ren with both assurance that the spirits have been laid to rest, and the song he was here to find.  He thanks you gladly for both gifts, and inquires if you thought to read the song, curious what it might mean- both in general, and to you.  Finally, turn to his daughter Ling, and offer her the Waters.  She tells you that she is glad to have them, but they are not a permanent cure- they will only grant the old panda some more time.

Spontaneously, a new quest will appear in your list- it tells you that you have done what you can for Anduin’s new friends, and your mission is complete: you’ve found him and need to return him to the SI:7 agents and the Admiral Taylor.  You must return home- and you must tell Anduin that it is time he do so as well.

When you speak to Anduin, he tells you that Ren has promised to show the boy the pandaren village his family lives in.  You will need to tell Anduin that it is time to go home.

When you do, he says that he cannot return yet- he wants to find the Vale, study the Sacred Pools, and learn of their healing powers.  Immediately after, Sully and Amber arrive, the dwarf telling him that it’s time to go home and he should come quietly along.  Anduin refuses, though- and hits Sully with the Priest spell Mind Control.  Under the Prince’s sway, Sully agrees that Prince Anduin is free to go, and the boy makes good time away from the camp with his pandaren friends, leaving you and the SI:7 agents behind.