From mortal +‎ -ness.

mortalness (uncountable)

1. The quality of being mortal; mortality.
   • 1852, Herman Melville, Pierre; or The Ambiguities:

     […] therefore, to him, Isabel wholly soared out of the realms of mortalness, and for him became transfigured in the highest heaven of uncorrupted Love.

• “mortalness”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.

• Last Sermon, marlstones