According to Obadiah, what had deceived Edom?

Answer: The pride of its heart

"The pride of your heart has deceived you, O dwellers in the clefts of the rocks," who said, "Who can bring me down?"

“The pride of your heart has deceived you, O dwellers in the clefts of the rocks whose habitation is the heights, who say in your heart, ‘Who can bring me down to the ground?’”— Obadiah 1:3 (BSB)