The Riddle of the Temple

Question

You enter an old temple, from a long-dead religion. Before you, in front of a pile of riches, stands a statue of a creature made of a mineral dark as night: a woman's head, a lion's body, an eagle's wings outstretched to block your path. In front of it, a plaque written in old language -- but thankfully, you thought ahead to bring along a Universal Translator Mk. 7000! The plaque reads as follows:

Make a promise, if you'd like. But keep your word, else I may strike.

A set of words may do you well, a phrase with which you cast this spell.

Letters all, but not once each, this phrase to me you must beseech.

What do you say, to get your reward?

Answer 1

I assume we are supposed to say

SPHINX OF BLACK QUARTZ, JUDGE MY VOW

which is

a famous pangram (but has two Os and therefore doesn't have just one of each letter)

although

this is pretty darn weird since evidently this thing's original language is not English

and

it seems like one could come up with any number of "not once each" pangrams that mention making and keeping promises, so this feels a bit too guess-what-I-was-thinking-y for my taste...

In case it's not obvious from the above,

there are some hints embedded in the story leading to this specific famous pangram. Woman's head + lion's body + eagle's wings = sphinx; mineral dark as night = black quartz; "make a promise" -- "my vow"; "else I may strike" -- an invitation to "judge".