"All Things Is/Are Ephemeral"
The subject "All things" is plural, ergo the verb is plural.
The complement "ephemeral" is an adjective, and so does not inflect for number in English anyway; but even if it were a singular noun, as "All things are a nuisance", "are" would still agree with "all things".
Because the word "things" is plural. This is the phenomenon known as case agreement, in its grammatical number form.