The Umbra Undertow was originally a pantheon dedicated to and supported by the Kuo-toa and Sahuagin. Much of the conflict between these two species can be linked to theological disputes over specific details of the Umbra Undertow. The Umbra Undertow started small, but the pantheons from civilizations lost to the sea are sometimes brought into the fold.
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There was an influx of new gods to the Umbra Undertow seven hundred years ago. Gods targeted by the Celestial Reclamation realized they had a choice. Be enslaved/consumed by the Celestial Reclamation, or drown their own civilizations to join their powers with that of the Umbra Undertow. The Celestial Reclamation was much more direct before the Great Floods. Now, the Celestial Reclamation works primarily through secrecy to achieve their ends of subjugating other gods for power.
Some of the newer worshipers of the Umbra Undertow are close to shore as they are still in their original positions. Other newer worshipers are much deeper as the landmass they inhabited were literally flung into the oceans to both ensure they would become submerged and displace enough water to drown portions of the surface world.
Many Aquatic Elves, Merfolk, and Tritons are followers of the Umbra Undertow. That said, many follow other pantheons instead. This holds particularly true for Aquatic Elves living in communities separate from the other aquatic races. The Elven tie to the Umbra Undertow came about when Lekamith left the Elven pantheon to join the Umbra Undertow. Some Aquatic Elves consider Lekamith both a traitor and a coward while others consider his decision as the only logical option and he is the savior of the Aquatic Elves.