unrelenting

Unrelenting, a.

Not relenting; unyielding; rigid; hard; stern; cruel. -- Un`re*lent"ing*ly, adv. -- Un`re*lent"ing*ness, n.

Shades of meaning

Fernald has no entry of its own for unrelenting; he lists it under severe.

Where he writes about it:

Austere signifies severely simple or temperate, strict in self-restraint or discipline, and similarly unrelenting toward others. — from severe