The adjective immutable means unchangeable, not capable or unable to be changed.

Synonyms are fixed, invariable, changeless, or inflexible.

The word origins from Old French (early 15th century) immutable (Modern French immuable) and directly from Latin immutabilis meaning “unable to change”. It is derived out of its assimilated form of in- meaning “not, opposite of” plus mutabilis which comes from the Latin verb mutare meaning “to change”. 

They all agreed that the rules should be immutable and unalterable.

It was designed to be immutable, no one can ever change it.

It was his fate, it was immutable.