Beseech the Water to Solidify quest
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The quest is given by Tomma, who mentions water sisters, aka Water Sprites. Head to Crossing and out the West gate to the Lake of Dreams. Walk around the area until you spot "some algae" (this will be in the "You also see" line, not in the room description). You may need to pass through the area several times before it shows up.
STUDY algae until you get "As you continue to study the algae you notice that it has more mass since you first began." You will receive a few more automated messages, ending with " Use it well and we hope that it will open your mind to knowing who and what are the real dangers out here."