All of you would know that the dragons of Lssthp
became the dragons of Lssthp after the Cataclym -- when Tiamat took offense at the independence of her children and tore the heart from the Plane of Fire, using it as a weapon to wipe out reptilian life in the continent of Daesic. The metallic survivors were those who had followed the prophet Bahamut out to sea some decades earlier; the chromatic survivors those who followed the Five Chromatic Gods to Lssthp just before the Cataclysm, in a betrayal of their vengeful tyrant mother. After the Cataclysm she was bound in the Fire Plane, beneath the central volcano of Sunhome Isle.
In the millennia that followed, the dragons and their servitor races took root in Lssthp, while the giants and their humanoid descendants claimed the wasteland that had been Daesic. The Five Gods were worshipped, not just in Lssthp, but in the human lands disguised as human gods.
Spoiler: Cult of Tiamat (5+)
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Many of you also would be aware of the Tiamatic cult that claims that the Five were obeying the will of their mother, or are in fact faces she wears, and that the long history of Lssthp and Daesic has all been according to her plan. The cult is illegal, but not precisely unheard of. There is much distance, however, between those who worship the Mother of Dragons, and those who would actively seek to free her.
Spoiler: Cult of Apophis (10+, or baseline for Yuan-Ti.)
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Fewer would be aware of the cult of Apophis, the snake god worshipped by the free Yuan-Ti of the Hic Sunt. Technically, only those serpentfolk who follow Apophis should claim the name Yuan-Ti. This faith is absolutely forbidden, as it is in many ways opposed to the Five Gods, but even in Lssthp, there are serpentfolk who serve Apophis by working against their draconic rulers in secret.
Spoiler: 15+
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These are far from the only secretive or forbidden religions of Lssthp. The most vile and mysterious are the worshippers of the Slaadi who ruled Lssthp before the dragons came.