For "immortal":
* Mriyu" (मृत्यु) - "death"
* Naashwan" (नाशवान) - "mortal", "perishable"
For "everlasting":
* Saamaanya" (साधारण) - "ordinary"
* Parivaar" (परिवर्तनशील) - "changeable"
* Aavartaan" (आवर्तन) - "temporary", "cyclic"
The best opposite word for "amar" will depend on the specific context of your sentence.