The Time of Troubles is a background element in the events of Baldur's Gate, Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Throne of Bhaal. It is the ultimate reasoning behind the entire plotline of these games.
The Time of Troubles was a period of great upheaval and turmoil in Faerûn, as Ao the Overgod, grew angry at the other gods and cast them from their homes in the Outer Planes to instead walk Toril as mortals. During these times, several gods perished, amongst them Bhaal, Myrkul and Mystra, and new gods were raised to take their place from mortals.
Bhaal foresaw the Time of Troubles, and took precautions beforehand in an effort to safeguard his portfolio. Taking on myriad mortal guises, he roamed Faerûn, siring sons and daughters amongst many different races; from humans to giants, orcs to elves. These children, known as Bhaalspawn, would bear a sliver of Bhaal's essence within them so, should Bhaal perish, then these children would die, and their souls would fuse together to recreate their father.
One of these Bhaalspawn, the ward of a sage named Gorion, rose beyond Bhaal's expectations. Whilst other Bhaalspawn came up with the idea of claiming Bhaal's throne for themselves, only Gorion's Ward would go on to achieve a true success...