Spear of Ithuriel
(The), the slightest touch of which exposed deceit. Thus when Ithuriel touched with his spear Satan squatting like a toad close to the ear of Eve, the “toad” instantly resumed the form of Satan.
“The acute pen of Lord Halles, which, like Ithuriel's spear, conjured so many shadows from Scottish history, dismissed among the rest those of Banquo and Fleance.” —Sir W. Scott.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894