1) Grim 2) Implacable 3) Inexorable 4) Relentless 5) Unrelenting 6) Voracious 1) Grim 2) Implacable 3) Relentless 4) Stern 5) Unforgiving 6) Unrelenting 
unappeasable 1. Not to be placated or appeased or moved by entreaty. 2. Incapable of being appeased, significantly changed, or mitigated. 3. Impossible to appease or to satisfy: 'We had an unappeasable thirst and hunger after the long, hot walk.'