Judges 21:5

And the children of Israel said
One to another, after they had offered their sacrifices, and while they were together in Shiloh:

who is there among all the tribes of Israel, that came not up with the
congregation unto the Lord?
when they were summoned to come to Mizpeh, to consult together about the affair of the Levite's concubine, as appears by what follows:

for they had made a great oath;
in a very awful and solemn manner, with a curse annexed to it, as that about not giving a wife to Benjamin, ( Judges 21:18 )

concerning him that came not up to the Lord to Mizpeh:
not about him who did not go out to battle against Benjamin, nor about every individual that did not come to consult about it; but every city that did not send their proper representatives or quota to assist in that affair:

he shall surely be put to death;
this was sent along with the summons, in order to quicken their attention to them.