5 Then the Israelites asked, “Who from all the tribes of Israel has failed to assemble before the LORD?” For they had taken a solemn oath that anyone who failed to assemble before the LORD at Mizpah was to be put to death.

Read Judges 21:5 Using Other Translations

And the children of Israel said, Who is there among all the tribes of Israel that came not up with the congregation unto the LORD? For they had made a great oath concerning him that came not up to the LORD to Mizpeh, saying, He shall surely be put to death.
And the people of Israel said, "Which of all the tribes of Israel did not come up in the assembly to the LORD?"For they had taken a great oath concerning him who did not come up to the LORD to Mizpah, saying, "He shall surely be put to death."
Then they said, “Who among the tribes of Israel did not join us at Mizpah when we held our assembly in the presence of the LORD ?” At that time they had taken a solemn oath in the LORD ’s presence, vowing that anyone who refused to come would be put to death.