When a mortal dies, her spirit waits for three days and nights on NUM. On each of the three nights, she contemplates: 1) the things she did, 2) the things she said, and 3) the things she thought. The spirit then travels through Schurn, the Underworld, to "The Bridge of Impartial Judgment", where she is going to get what she deserves.
If she was middle of the road in her life, neither noticeably good nor evil, she goes to the Emptiness, total oblivion. If the spirit was really good in life, she is met by a wizened old man who takes her by the hand and leads her across the bridge to the House of Song, an eternal paradise. The old man represents the spirit's conscience, which has seen much experience in the life of one who is good. That is why he is so old and decrepit.
If the spirit was evil in life, she is met by a beautiful young boy who signals to a pair of jenn, who take bladed flails and drive her onto the bridge, which is now like a sword blade turned on edge. She cuts herself up and falls right off into a vat of molten metal. She eventually ends up in the House of the Lie, a place of eternal torment.
The Zend-Avestites believe in a more forgiving afterlife, where the vat of molten metal purifies the soul, which comes out the other side clean and then enters House of Song.