According to human legends, how were the Tears of Selûne formed, and what role did the goddess Selûne play?
Here's what the lore says, drawn directly from the indexed sources:
Tears of Selûne > Tears of Selûne > Rumors & Legends The elves of Evermeet took the appearance of the Tears as a sign that the gods were pleased with their magical research and viewed the Tears with pride, while the goblinoids believed that the Tears were part of a plot by human mages to bathe the night in moonlight. The goblinoids feared that the night sky would soon be filled with moons, resulting in the elimination of their precious darkness. As a result, many goblinoids set out to kill as many humans and elves as they could due to this belief. Spacefarers tended to believe that the Tears…
Source: Tears of Selûne
Selûne > Selûne > The Dawn Cataclysm During the divine conflict known as the Dawn Cataclysm, Tyche, the goddess of luck, was covertly corrupted by Moander, the god of decay, whilst on her travels. She returned to her realm and found Selûne, a dear friend, had come to speak with her, as well as the god of the dawn Lathander, Tyche's ex-lover and instigator of the Dawn Cataclysm, and the god of mages, Azuth, who'd come to mediate. However, seeing the rot within Tyche, Selûne began to weep great tears and swiftly struck her with a bolt of light intended to purify her. However, Tyche instead…
Source: Selûne