21 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.

Read Isaiah 28:21 Using Other Translations

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.
For the LORD will rise up as on Mount Perazim; as in the Valley of Gibeon he will be roused; to do his deed--strange is his deed! and to work his work--alien is his work!
The LORD will come as he did against the Philistines at Mount Perazim and against the Amorites at Gibeon. He will come to do a strange thing; he will come to do an unusual deed: