General antonyms:
* Merciful
* Forgiving
* Lenient
* Compassionate
* Understanding
* Reconciliatory
* Appeasable
* Conciliatory
Antonyms specific to certain contexts:
* Yielding (to pressure)
* Flexible (in negotiations)
* Pliable (in opinion)
* Amenable (to reason)
The best antonym will depend on the specific context in which "implacable" is being used.