Chult is a special land (dinosaurs!) and so, apparently, it also deserves its own unique religion in the margin of the faerunian’s pantheon. Ubtao is the god of Chult. Or was. And, in fact, he’s not a god but a primordial, but whatever, it’s all very confusing. In Tomb of Annihilation, the module designers took the easy way out and Ubtao is simply absent. To be fair, the FR’s lore concerning him is a mess, as it often happens when the primary source is a (bad) novel (and several editions’s retconning doesn’t help either).
But what is, maybe, the most mind-boggling thing about Ubtao is that he’s both the Father of the Dinosaurs AND the one who invited humans to settle in Chult. I mean, how did that go? « Come to my land, I built a great city just for you. But, uh, don’t wander too much outside its walls ’cause my beloved children will eat your guts… »
(Also, I’m putting aside the Shadow Giant, Dendar the Night Serpent, all the Maze stuff and, most of all, Mezro the City in Heaven. Less confusion this way.)
Let’s just say that he’s a dual entity (like Janus or Hoar in Faerun), with a cycle where he’s active towards helping civilization, followed by a cycle where he’s more dissipated and all about claws and teeth and no pity for the weak.
What’s interesting with this take is: what if his devoted followers can nudge him, just a little maybe – with appropriate behavior, ritual, quest and whatnot – towards his next phase, whichever it is?