From severe + -ness.

severeness (uncountable)

1. The property of being severe.

   Everyone was surprised by the severeness of the sentence the judge imposed.

   • 1996, Chantal Joubert, Hans Bevers, Schengen Investigated, page 305:

     Depending on the severeness of the crime and the danger of the suspect, the apprehender may apply physical force in order to stop the pursued and proceed to his apprehension.

• severity