For they had made a great oath— See what has been said of this oath or anathema in the last note on the 19th chapter.
Related Commentaries of Judges 21:5
Judges 21:5
5 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.