unrelenting
adjective
1. not relenting; having no pity; not being or becoming lenient, mild, gentle, or merciful; unyielding.
Similar word(s): implacable, grim, inexorable, relentless, stern, unappeasable, unforgiving
2. punishingly harsh
Similar word(s): intense, brutal
3. never-ceasing
Similar word(s): continual, persistent, relentless