A massive Cataclysm happens around 100 years or so after BG takes place. During this span of events, several gods die, several new gods emerge and the land is all torn up.
It was meant to "reboot" the Forgotten Realms setting.
During this "time of troubles" 3 human beings actually ascended to godhood to take the place of the fallen gods. (Mystra, Cyric and Kelemvor; Magic, Murder/Strife and Death respectively)
Yup, the God at the centre of the BG series dies, and is replaced by one of the most insidious god figures in the entire Forgotten realms, the Prince of Lies.
For these fantastic tales see "The Avatar Trilogy, The prince of lies, and The trial of Cyric the mad", great books, I highly recommend them.