'unrelenting sorrow' is correct and usable in written English. You could use it to describe someone's persistently sad state, for example: "As the weeks went by, his unrelenting sorrow showed no signs of abating.".
It fosters a peaceful reckoning and loving coexistence but equally tempestuous anger and unrelenting sorrow.
Chanting, unrelenting.
The pace was unrelenting.
Her headache was unrelenting.
Everywhere, all over, unrelenting".
Day after unrelenting day.
defense that was unrelenting.
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia