Example : Grim determination.
Grim necessity.
Russia's final hour, it seemed, approached with inexorable certainty.
Relentless persecution.
The stern demands of parenthood.

Synonyms : grim, relentless, stern, unappeasable, unforgiving, unrelenting

Example : He is adamant in his refusal to change his mind.
Cynthia was inexorable; she would have none of him.
An intransigent conservative opposed to every liberal tendency.

Synonyms : adamant, adamantine, intransigent