Old Slavic religion evolved over more than a thousand years and some parts of it were from neolithic or possibly even mesolithic times. Little could be said about any of its practices that would be valid during this entire period. It is thought that the Book of Veles is the sacred text of this religion.
It recognized three worlds: Jav, Prav and Nav, Jav being the material world, Nav the immaterial and Prav being the laws that govern them.
The supreme god of Slavic pantheon is Svarog a.k.a. Triglav. But exactly because of his nature he was not the most worshiped: that was Perun, while tribes that were occupied mostly with cattle could pay most attention to Veles and so on.
Underworld of this religion was Svarga, residence of Svarog, and Irij was its paradise.
In addition to the gods, the pantheon also includes a number of lesser deities.
These heroes are based on real persons but most stories about them became pure fiction, involving magic etc.