כִּ֤י כְהַר־פְּרָצִים֙ יָק֣וּם יְהוָ֔ה כְּעֵ֖מֶק בְּגִבְע֣וֹן יִרְגָּ֑ז לַעֲשׂ֤וֹת מַעֲשֵׂ֙הוּ֙ זָ֣ר מַעֲשֵׂ֔הוּ וְלַֽעֲבֹד֙ עֲבֹ֣דָת֔וֹ נָכְרִיָּ֖ה עֲבֹדָתֽוֹ׃
For the LORD will rise up as in mount Perazim, He will be wroth as in the valley of Gibeon; That He may do His work, strange is His work, And bring to pass His act, strange is His act.
Read Kabbalistic (mystical) interpretations of Isaiah 28:21: Zohar and classical Jewish mystical sources, free online.