The 1905 LDS edition accidentally set the plural swords instead of the singular sword. This typo was corrected in the 1920 LDS edition. Elsewhere the Book of Mormon refers only to “the sword of justice” (eight times), including later on in this very same verse:
Included in the eight examples is 1 Nephi 12:18, which in the original manuscript reads as “and a great and a terrible gulf divideth them yea even the sword of the justice of the eternal God” (that is, not as “the word of the justice of the eternal God”, the reading of the current text). One other passage also supports the singular sword when referring to God’s judgment:
Thus the original singular usage in Helaman 13:5 is definitely correct.
Summary: In the Book of Mormon, the noun sword always takes the singular number when referring to God’s judgment, including twice in Helaman 13:5.