Keening is a legendary Short Blade crafted by the Dwemer Tonal Architect Lord Kagrenac. Together with Sunder and Wraithguard, these tools are necessary to handle the Heart of Lorkhan. All three artifacts came into the possession of the Almsivi following the Battle of Red Mountain. Keening, along with Sunder, was subsequently lost during an annual pilgrimage to the Heart, sometime in the Third Era.
Keening is obtained during the end of the Main Quest and is required to finish it. Though available at any time, it can't be equipped due to the 'mortal wound' ability. Once you have Wraithguard equipped, Keening may be used. See the Keening Artifact Quest for more information.