Just clarifying the use of what I take to be the fut. perf.
The kings are asking the two sets of triplets to fight it out to decide which of the two tribes shall rule - the Romans or the Albani (this is Orberg’s cap XLIII adapted from Livy)
Eos rogant reges ut pro sua quisque patria dimicet ferro: ‘ibi imperium fore unde victoria fuerit’.
The king asked each of them would they fight with arms for their country: ‘here is to be government from the victory that shall have been.’
I’m trying to translate this as literally as I can - so it’s not elegant. fuerit is a future perfect 3rd pers. sing. of the verb esse and my understanding is that it means ‘shall have been’. Do I have this right or does the future perfect have a more idiomatic sense than I am aware of?
fuerit here is perfect subjunctive, representing what in direct speech would be future perfect (which would, however, be identical in form–fuerit).
It follows the normal sequence of tenses for primary main verbs. Although this is in the past, rogant is (historical) present, and the understood main verb ("they agree that . . . ") on which the indirect speech (ibi imperium . . . fuerit) depends is treated as a present, i.e., primary, tense verb.
In direct speech it would be “ibi imperium erit under victoria fuerit.” “Dominion will be there whence the victory will have come [literally, “been”].” In indirect speech, the perfect is used because the victory happens before dominion is established.
My understanding was that perf. subjunctive and fut. perf were the same - except for the 1st person singular. I don’t understand why it’s perf. subjunctive but you’re translating it as, in essence, a fut. perf. - shall have been?
It’s a subordinate clause in indirect speech, so it must be subjunctive. It can’t be future perfect indicative.
But the sequence of tenses requires that verb that occurred before the main verb (the verb in the infinitive) in indirect speech be in the perfect tense where the verb of speech that introduces the indirect speech is in a primary tense.
Here, the speech verb is understood, but the whole sentence is historical present. So you can assume that the understood speech verb is in the present tense, triggering the primary tense sequence of tenses.