Deepness
The Deepness was an evil blight on the world of Scadrial pre-Ascension.[1]
It was caused by Ruin encouraging the Snapping effects of the mist to kill people more than it normally did. The skaa feared the mists, as there are also creatures such as mistwraiths and the Allomatic Snapping which killed many.[2]
The Deepness was characterized as some formless enemy, which the Lord Ruler defeated prior to his Ascension. During the time of the Mistborn trilogy it was unknown what form the Deepness took when it was first defeated by The Lord Ruler.
The Deepness was in fact the mists. They 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, and the Lord Ruler harnessed the power of the Well of Ascension to defeat the Deepness, which had a great number of unintended consequences.