KJV:
For the LORD shall rise up as in mount Perazim, he shall be wroth as in the valley of Gibeon, that he may do his work, his strange work; and bring to pass his act, his strange act.
NKJV:
For the LORD will rise up as at Mount Perazim, He will be angry as in the Valley of Gibeon, That He may do His work, His awesome work, And bring to pass His act, His unusual act.
NIV:
The LORD will rise up as he did at Mount Perazim, he will rouse himself as in the Valley of Gibeon, to do his work, his strange work, and perform his task, his alien task.
Interpretation:
The Lord will rise up to do His strange work and bring to pass His strange act. God's strange work refers to judgment, which seems unusual because God is normally a God of mercy. The verse shows that God will judge when necessary. His judgment work is strange because it goes against His normal character of mercy. The verse teaches that God sometimes does strange things. Those who face God's strange work experience His judgment. The verse reminds us that God will judge when people persist in sin.