unmerciless

From un- +‎ merciless (with un- functioning as an intensifier of -less).

unmerciless (comparative more unmerciless, superlative most unmerciless)

1. (obsolete) Utterly merciless.
   • 1545, George Joye, The exposicion of Daniel the Prophete […] :

     Now ye see […] their deceites, vnmerciles murther, and ingratitude.

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