(n.) The quality of being implacable; implacability.

acerbity acid acidity acidulousness acrimony animosity animus antagonism antipathy asperity bad blood bad feeling bile bitter feeling bitter resentment bitterness bitterness of spirit causticity choler enmity feud gall gnashing of teeth grudgefulness hard feelings hate hatred heartburning hostility ill blood ill feeling ill will implacability implacableness irreconcilableness malevolence malice malignity rankling resentfulness resentment revengefulness slow burn soreness sourness spite spleen unappeasableness vendetta vengefulness venom venomousness vindictiveness virulence vitriol