וַיְשַׁלַּ֨ח יְהוָ֜ה בָּעָ֗ם אֵ֚ת הַנְּחָשִׁ֣ים הַשְּׂרָפִ֔ים וַֽיְנַשְּׁכ֖וּ אֶת־הָעָ֑ם וַיָּ֥מָת עַם־רָ֖ב מִיִּשְׂרָאֵֽל׃
And the LORD sent fiery serpents among the people, and they bit the people; and much people of Israel died.
Read kabbalistic (mystical) interpretation of Numbers 21:6: and classical mystical sources.