Once the ritual has been performed, you can summon the soul again using the sheer power of your own voice.

Return to the Hermit Monument to summon Zagus.

Ask him once more about the Treasure of the Ancient Kings.
(This is the Hermit Monument.)

(It seems to call your name.)
I've been expecting you, outlander.

You may only have just summoned me, but my call rang out to you long ago.

You seem to me to have a trustworthy heart.

Which is why I'd like to ask you for a favour.
I have seen you talking to another Aquan.

If it was about the Treasure of the Ancient Kings, then I can help you.

Go to the place I tell you and search for the Settler's Chest.

You will find a clue there to the location of the Treasure of the Ancient Kings.

But it could be dangerous for you.

You should not trust the other Aquans; but call only on your own allies for help.
(It seems to be Zagus' Box.)

(You open it up and find a map inside.)
(This box is right at the place marked on the map.)

(It may be the Treasure Chest.)

(But it could also be a trap.)
So that's what happened.

I'll tell Soglo about this.

What an interesting legend. The ancient kings...

However, I wonder if this Treasure of the Ancient Kings of the Aquans actually exists.