My research indicates that the gods who were once mortals include the following:
- Azuth was once the mortal champion of Mystra and ascended to godhood after imprisoning Savras.
- Bane, Bhaal, and Myrkul (also known as the Dead Three) became gods when Jergal gave up his powers.
- Cyric once was a thief, but ascended to godhood during the Time of Troubles by slaying Bhaal.
- Finder Wyvernspur was once an accomplished bard and (after some trouble) killed Moander and claimed his divine spark.
- Gwaeron Windstrom was made a demigod by Mielikki after he killed an avatar and several manifestations of Malar.
- Kelemvor was once a human fighter named Kelemvor Lyonsbane. He became a god after defeating Cyric.
- When the previous Mystra was destroyed by Helm, Ao replaced her with Midnight, a powerful wizard.
- Savras once was a powerful mage who somehow became a god.
- Torm was once a mortal hero and became a demigod during the time when Netheril fell.
- Valkur may have once been mortal.
Are there others? What's the full list of Forgotten Realms deities that were once mortal?
Also, all the deities I've found were humans before their ascensions. Have any nonhumans become deities?