Consider the following aphorism by Nietzsche:
Ich liebe Die, welche nicht zu leben wissen, es sei denn als Untergehende, denn es sind die Hinübergehenden.
I unterstand that the cluster es sei denn means unless, however, in the last sub-sentence, why is sind used if, apparently, the subject is es? Should one not use sei instead?
Es sei denn is a fixed phrase that does not influence what follows; especially, there is no need for it to be followed by the present subjunctive. Note that sind isn't present subjunctive, but indicative; the former would be seien.
The question remains why singular es is followed by plural sind. This happens when es is the subject and a plural noun phrase is the subject complement.
Schau dir mal die Fotos an. Es (subject) sind einige meiner besten Aufnahmen (subject complement).
Note that the grammatical structure changes completely in cases such as the following.
Schau dir mal die Fotos an. Es (expletive) sind einige meiner besten Aufnahmen (subject) darunter (subject complement).
The expletive es has no grammatical function and disappears when the sentence is rearranged.
Schau dir mal die Fotos an. Einige meiner besten Aufnahmen sind darunter.
That es is the same kind of it as in the English phrase It rains.
You need it for grammar reasons but it's not the subject of that clause. The real subject is die Hinübergehenden, which is plural. Hence es sind, not es ist.
Now you can ask why it's not es seien. This is because that part of the sentence is neither stated as hearsay nor as an assignment. But as a fact.
That es sei denn in the previous clause isn't an assignment either. It's a pre-packaged phrase meaning unless.
