Night of Souls
From Diablo Wiki
The Night of Souls is a recurring event on Sanctuary where souls of the dead are in flux.
Whether it's once per year, or once every couple of years is currently unknown.
According to Tyrael, it is said that in some places the restless dead return during the Night of Souls. Some return to take vengeance against those who had slain them. Some come to see their loved ones again. And others just can't rest. Sometimes it is the earth itself that brings them back, remembering the life force that once was.
Life does not simply give in to death, and the soul is more than some abstract idea.