From the developers:
The events between Baldur’s Gate: Enhanced Edition and Baldur’s Gate II: Enhanced Edition are at last revealed in this exciting new expansion to the original game.
Though Sarevok is dead and his plan for war averted, peace eludes the citizens of Baldur’s Gate. A crusade marches in the north, seizing supplies, forcing locals into military service, and disrupting trade along the Sword Coast. A charismatic warrior known as the Shining Lady leads this army, her background shrouded in mystery. Can the rumors be true—is she, like you, the child of a god?
The closer you get to the Shining Lady, the more you realize your father, the dead Lord of Murder still casts a long shadow upon your path. Baldur’s Gate has put its faith in you, but you must determine whose interests you truly serve before you face the Shining Lady among the ruins of Dragonspear Castle…
There's no release date set for Baldur's Gate: Siege of Dragonspear, but will take place between Baldur's Gate 1 and 2, after the death of Sarevok, but before Irenicus.