Example sentences:

Ramayana:  Vishwamitra reported that the demons were becoming implacable; the more people retreated from the territory of the demons, the more the demons expanded their territory, devouring humans and destroying human habitats.

Mahabharata:  Duryodhana’s implacable enmity toward the Pandavas arose from his own envy and insecurity, not from anything the Pandavas had done.

Bhagavad-gita:  The Gita cautions that lust can be an implacable inner enemy; the more it is pandered, the more it demands till it consumes our entire life, lifetime after lifetime.