implacable
Entry from World dictionary
Pronunciation:/ɪmˈplakəb(ə)l/
adjective
- unable to be appeased or placated:he was an implacable enemy of Ted's
- unable to be stopped ; relentless:the implacable advance of the enemy

Derivatives
implacability
Pronunciation:/-ˈbɪlɪti/nounimplacably
adverb

Origin:
late Middle English: from Latin implacabilis, from in- 'not' + placabilis (see placable)