1. grim, inexorable, relentless, stern, unappeasable, unforgiving, unrelenting

not to be placated or appeased or moved by entreaty

Example Sentences:
'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

2. brutal, unrelenting

harsh

Example Sentences:
'the brutal summer sun'
'a brutal winter'

Synonyms

• brutal

3. persistent, relentless, unrelenting

never-ceasing

Example Sentences:
'the relentless beat of the drums'

Synonyms

• persistent
• relentless