La Profundidad
The Deepness was an evil blight on the world of Scadrial prior to the Ascension of the Lord Ruler.[1]
Pre-Ascension
The deepness was the name for when Ruin encouraged the mists to cover the land, completely causing plant life to die off.[2] This presented the world with a threat that couldn't be fought with traditional means. The mists were growing in strength and crops were unable to grow. The threat of starvation faced the world. When Rashek harnessed the power of the Well of Ascension to defeat the Deepness, he first moved the planet closer to the sun to burn it off. This caused numerous problems that Rashek had to scramble to fix because Rashek did not have the knowledge he needed to do it right.
Post-Ascension
The Deepness was depicted as some formless enemy that the Lord Ruler defeated prior to his Ascension. During the time of the Mistborn trilogy, it was unknown what the Deepness was when it was first defeated by The Lord Ruler. With the knowledge that the Night time mists are the deepness, the Skaa's Fear of the Mist has a greater foundation. The historical snapping by the mist during the Pre-Ascension lends some credit to the Skaa's fear that the mists will steal their soul. This, combined with the fact that Mistwraiths only came out at night, helped to solidify the Skaa fear of the mists. After Ruin's release he brought the mists back during the day, smothering the planet once again.
See Mist for more information on the Mist's composition.