Question
In the middle of Act 1, to get access to the sunken temple, you have to go in the field where the Nephalem, if I understand it well, killed each other until there was not enough to continue the race. Their ghosts were still fighting in that very field.
But what triggered that "Civil war" of the Nephalem? What happened to make them start to kill each other to a point where their species just disappeared from Sanctuary?
Answer
The Nephalem were the direct offspring of demons and angels, and had the potential to exceed both of their progenitors in power. Fearing this, Inarius manipulated the Worldstone so that the Nephalem would slowly lose their power over many generations, causing them to become the humans who populate the world today. The Nephalem didn't really disappear, they just lost their powers and became human.
The Sin War, as it is called, was merely a name for the extension of the Great Conflict between Light and Darkness into the mortal world of Sanctuary. Both sides manipulated the Nephalem to their own ends to try and gain the upper hand in the conflict. (Eventually both sides agreed to cease this practice, but the forces of Darkness had a plan...)
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