Tyche, goddess of fortune, was originally a Titan, a daughter of Oceanus and Tethys. She sided with the Olympian gods against the majority of the titans, and thus gained an honored place on Mount Olympus.
Some say she was the sister of Nike and Eris and had a falling out with them, and since then they have worked to rid themselves of her. Whatever the truth of the matter, she has played less and less of a role in the pantheon over the millennia.
She fled to the world of Toril, where she was infected by the evil essence of Moander, and split into two seperate goddesses by Selûne.
Now she takes the form of Tymora and Beshaba, good luck and bad. They have seperate personalities and consider themselves to be bitter enemies. Lathander recently tried to put her back together, without success (see the novel Tymora's Luck). Hermes and Pan have both tried to send avatars to Toril in search of her, but thus far they have been barred by Ao from finding the truth.
Tyche's Realm in Olympus is slowly fading. In ages past, it was a grand villa and gambling hall, but now it's desolate and abandoned, full of cobwebs and dust.
Sources:
Deities & Demigods, 1st edition, page 69
On Hallowed Ground, pages 127-128
Tymora's LuckDeities & Demigods, 3rd edition, pages 130-132
Netheril: Empire of Magic, The Winds of Netheril, pages 60-63
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