The Time of Troubles
The Time of Troubles, also known as the Arrival, the Godswar and the Avatar Crisis, was a cataclysmic time period in the chronology of Faerûn. Taking place during 1358 DR, the Year of Shadows, the Time of Troubles was a period during which the deities of Faerûn were forced to walk the earth in their mortal avatar forms. Several major deities died during the Time of Troubles (see Deaths, Ascensions, and Resurrections) and a handful of mortals rose to divinity.
The Time of Troubles was precipitated by an attempt by the gods Bane and Myrkul to steal the Tablets of Fate from the overdeity Ao. Angry at the gods for their habitual pursuit of power and negligence toward their mortal faithful, Lord Ao relegated every god (except for the guardian god Helm, selected to protect the gates to the heavens) to walk among their followers on the earth.
The immediate effects of this edict were threefold. First, divine magic (spells granted to clerics by their patron deities) ceased to function altogether unless the cleric was within one mile of their deity’s avatar. Second, arcane magic (a force channeled from the Weave by wizards and sorcerers) ceased to be regulated by its steward, Mystra and became dangerously unpredictable. Third, the characteristically immortal and aloof deities were now vulnerable (though still devastatingly powerful) and dwelling among the civilizations of Faerûn.
The Deaths and Ascensions of Gods
As a result of their mortality, several god were slain during this turbulent time and several mortals ascended to take their places. Your group have heard of several of these incidents in particular.
Bane, greater god of tyranny, strife and hatred, was slain on Eleasias 13, in a climactic battle with the demigod Torm in battle outside of Tantras. Though he was annihilated by Bane’s dying breath. Torm was later resurrected by Ao because he died fulfilling the obligations of his portfolio.
Bhaal Lord of Murder was slain by the young mortal Cyric with the sword Godsbane on the Boareskyr Bridge. His blood, along with the shattered fragments of his sword, spilled into the Winding Waters, corrupting miles of the great river with his essence.
Mystra the Lady of Mysteries and the Mother of All Magic was slayen by Helm for attempting to reclame her place in the planes. One of her most devout and talented followers, Midnight, then ascended to take her place and restore the Weave.
The adventurer Cyric absorbed both Bane and Bhaal’s portfolios and ascended to godhood as the Prince of Lies, the Dark Sun and Lord of Death.
In the years following the crisis, the mortal Kelemvor was able to steal the portfolio of Death and the Dead from Cyric and is now the most recent god to retain the title of Lord of the Dead.