What is the definition of the phrase "unto itself"?

I've looked through several online dictionaries to discover the meaning of the phrase "unto itself" ("unto itself" in the sense of the phrase having no qualifying objects, people, or concepts to take the place of the -sth/-sb). But alas, all my efforts at discovering the original definition of the preceding idiomatic phrase are to no avail, as no online dictionary provides me with an adequate explanation. I have been successful in searching for the meanings of its derivatives/variations.

Take Collins Dictionary, for example:

A law unto yourself: "If you say that someone is a law unto himself or herself, you mean that they behave in an independent way, ignoring laws, rules, or conventional ways of doing things".

Unto: "If you say that something is, for example, a world unto itself or a place unto itself, you mean that it has special qualities that it does not share with other, similar things, and so it should be treated or understood differently from those other things".

Other variations include a world unto itself, an island unto itself, a means unto itself, an entity unto itself, a world wholly unto itself, sufficient unto itself***

As I've stated in the introduction, I've been unsuccessful in my bid to discover the one true underlying definition for this elusive idiomatic phrase. Whatever definition I do search for is relegated to only its derivatives; this I find most unsatisfactory in passing my muster.