Thunder Blessing

The Thunder Blessing, also known as the Thundering, occurred in the Year of Thunder, 1306 DR, and was the dwarven god Moradin's gift to his people which dramatically increased the reproductive rate of dwarves. Moradin called a council of the dwarven pantheon to find a way to increase the number of dwarves. The council decided to split each existing soul into two when they were reincarnated. After the Thunder Blessing most dwarven births were twins.