Relentless
RELENT'LESS, adjective Unmoved by pity unpitying; insensible to the distress of others; destitute of tenderness; as a pray to relentless despotism.
For this th' avenging pow'r employs his darts,
Thus will persist relentless in his ire.
Relentless thoughts, in Milton, may signify unremitted, intently fixed on disquieting objects.
[This sense of the word is unusual and not to be countenanced.]