Heler is the rain that accompanies Orlanth Thunderous. He is the water that falls from the Air. He is a minor deity in most of Dragon Pass as his blessings can be obtained just as easily through Orlanth. In general, the Sartarites view Heler as an accoutrement of Orlanth, like the Four Weapons cult. Where he is worshiped outside of Orlanth, it is usually in conjunction with Engizi. There are perhaps 2600 initiates of Heler/Engizi in all of Sartar, about a third of them are ducks.
Heler’s cult is much larger in Esrolia – nearly 12,000 people. But that is more a result of just how populated Esrolia is! Esrolia is populated enough, and wealthy enough to support a large cult of the Rain God.
That of course has social and magical implications – thousands of Rain God devotees might gather at big religious ceremonies that call down the rain. They are numerous enough that they can do this on their own.
But Orlanth Thunderous also has a big cult in Esrolia. Heck, there are over 200k people that are devoted to some aspect of Orlanth, including Barntar.
