Corruption appears to have been born from the destruction caused by the Nephilim but made real by the hatred and agony of the first Nephilim, Absalom after he met his end at Death's scythe. After his defeat at Death's hand at Eden, a black pool formed beneath him. As he lay dying, he was consumed by the pool, despite Death's attempts to pull him out.
Now Corruption spreads throughout all Creation, seeking to undo the balance and plunge all of Creation into nothingness with only the Balance to stop it. - Darksiders Wiki