I had originally provided a comment to an answer (which is now deleted) saying that all can be either singular or plural. But that's not really correct. As a comment, I didn't have space to explain it properly.
All is used along with a subject that is either singular or plural.
I ate (all / most / some / part) of the cake.
Sometimes, all is used in a sentence with a subject that's considered singular, and sometimes in a sentence with a subject that's considered plural.
That is all.
All of the cars are white.
According to Merriam-Webster, all can be an adjective, adverb, or pronoun, and it can be used in sentences that involve singular nouns or plural nouns.
It can also be a noun itself. When it is, it's considered to be singular.
Whether it's used in a sentence with a singular subject or a plural subject is contextual.
All is well.
Here, it's used in a singular construction. It's being used in the same sense as everything would be:
Everything is well.
On the other hand, this would also be acceptable, if not common:
All (of them) are well.
In this case (assuming of them is not actually present in the sentence), all is being used as a pronoun in place of something like:
Things are well.
People are well.
Or, depending on how you look at it, it's not a pronoun but an adjective for something that simply isn't present:
All things are well.
All people are well.
All hail Caesar!
The verb form really has nothing to do with a singular or plural subject in this sentence.
What this sentence is actually saying is:
All (of you should / must) hail Caesar!
While the sentence is implicitly talking about the plural you, hail is the conjugation used for both the singular and plural form of you.