הִנֵּ֤ה אָֽנֹכִי֙ שֹׁלֵ֣חַ לָכֶ֔ם אֵ֖ת אֵלִיָּ֣ה הַנָּבִ֑יא לִפְנֵ֗י בּ֚וֹא י֣וֹם יְהוָ֔ה הַגָּד֖וֹל וְהַנּוֹרָֽא׃
Behold, I will send you Elijah the prophet Before the coming Of the great and terrible day of the LORD.
Read kabbalistic (mystical) interpretation of Malachi 3:23: and classical mystical sources.