La Profundidad
The Deepness was considered an evil blight on the world of Scadrial prior to the Ascension of the Lord Ruler.[1] In reality, it was the mist banks, created by Preservation, used to Snap the people of Scadrial in anticipation of Ruin's release.
Pre-Ascension
During the time before the Final Empire, when the Well of Ascension was about to become full, Ruin's influence in the world increased. He encouraged the mists to cover the land during the day, blocking off sunlight and causing plant life to die off.[2] The mists also started attacking people, which was Preservation's way of Snapping potential Allomancers, although some people died in the process. With crops unable to grow and the mists apparently killing people directly, this presented the world with a threat that could not be fought with traditional means. This phenomenon became known as the Deepness.
When Rashek harnessed the power of the Well of Ascension, he moved the planet closer to the sun in an attempt to burn the Deepness off. This caused numerous problems that led to the ash-ridden world of the Final Empire.
Post-Ascension
The mists returned to normal soon after, only coming out at night and remaining harmless. After many years, details of what happened became lost to history as Rashek's new role as the Lord Ruler was solidified and his original identity hidden. The Steel Ministry depicted the Deepness as a formless enemy that the Lord Ruler defeated prior to his Ascension. Few people knew that the Deepness was the same as the nightly mists, although many Skaa still fear to go out at night because of frightening stories told from generation to generation about how the mists could steal people's souls. Most nobles, particularly Allomancers and especially Mistborn, were unafraid of the mists.
After Ruin was released from his prison, he once again made the mists remain during the day. This caused the same problems it once did, making people realize what the Deepness actually was.