וְהֵיתָ֙יִת֙ אֶ֣בֶן חֲדָ֔ה וְשֻׂמַ֖ת עַל־פֻּ֣ם גֻּבָּ֑א וְחַתְמַ֨הּ מַלְכָּ֜א בְּעִזְקְתֵ֗הּ וּבְעִזְקָת֙ רַבְרְבָנ֔וֹהִי דִּ֛י לָא־תִשְׁנֵ֥א צְב֖וּ בְּדָנִיֵּֽאל׃
And a stone was brought, and laid upon the mouth of the den; and the king sealed it with his own signet, and with the signet of his lords; that nothing might be changed concerning Daniel.
Read Kabbalistic (mystical) interpretations of Daniel 6:18: Zohar and classical Jewish mystical sources, free online.