Relentless, a.
Unmoved by appeals for sympathy or forgiveness; insensible to the distresses of others; destitute of tenderness; unrelenting; unyielding; unpitying; as, a prey to relentless despotism. For this the avenging power employs his darts,.. Thus will persist, relentless in his ire. Dryden. -- Re*lent"less*ly, adv. -- Re*lent"less*ness, n.
Shades of meaning
Fernald has no entry of its own for relentless; he lists it under severe.
He lists it without comment; the entries above explain its neighbors.