Re·lent·less, a. Unmoved by appeals for sympathy or forgiveness; insensible to the distresses of others; destitute of tenderness; unrelenting; unyielding; unpitying; as, a prey to relentless despotism.
  For this the avenging power employs his darts, . . .
  Thus will persist, relentless in his ire.   --Dryden.
  -- Re*lent*less*ly, adv. -- Re*lent*less*ness, n.
  ◄ ►
  relentlessness
       n : mercilessness characterized by an unwillingness to relent or
           let up; "the relentlessness or their pursuit" [syn: inexorability,
            inexorableness]