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

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

Example : The brutal summer sun.
A brutal winter.

Synonyms : brutal

Example : The relentless beat of the drums.

Synonyms : persistent, relentless