The Phandelver Irregulars
A Primordial race, the Batrachi lived in the seas and oceans. As a sea creature, they developed an innate ability to camouflage themselves as they hid on the sea floor. Their ability to change their look eventually helped them form sentience. They gained the ability to use magic by corrupting the Sarrakh. After the Sarrakh fell into civil war, the Batrachi built a large empire.
A primordial race that created a vast empire after the destruction of the Sarrakh. While developing magic, accidentally opened a portal to the Astral Plane. Fought a long battle against the Titans. Weakened, the race was unable to handle the environmental changes caused by the Tearfall.
While there are ruins of their civilization, this race no longer exists. Other amphibian races have stories of their creation by the Batrachi.