וַיֶּחֱרַ֨ד יִצְחָ֣ק חֲרָדָה֮ גְּדֹלָ֣ה עַד־מְאֹד֒ וַיֹּ֡אמֶר מִֽי־אֵפ֡וֹא ה֣וּא הַצָּֽד־צַיִד֩ וַיָּ֨בֵא לִ֜י וָאֹכַ֥ל מִכֹּ֛ל בְּטֶ֥רֶם תָּב֖וֹא וָאֲבָרֲכֵ֑הוּ גַּם־בָּר֖וּךְ יִהְיֶֽה׃
At labis na labis na nanggilalas si Isaac, at sinabi niya: “Sino nga, at nasaan, ang nanghuli ng hayop at nagdala nito sa akin? Kumain ako mula sa lahat bago ka dumating, at binasbasan ko siya. Oo, at siya'y magiging pinagpala!”
Rashi
ויחרד [AND ISAAC] TREMBLED — Explain it as the Targum renders it ותוה, which means he was perplexed. The Midrashic explanation (Midrash Tanchuma, Toldot 11) is that he saw Gehinnom opening beneath him (Esau).
Chizkuni
ויחרד יצחק, “Yitzchok became frightened;” according to Rashi, he had a vision of gehinom opening up beneath him. He had this vision as soon as he wanted to curse Yaakov for having deceived him. Some commentators understand the expression as Yitzchok now realising that if he could not even have known the difference between his two sons he could no longer be surprised why he had been struck with blindness as a punishment. Nonetheless, there was one thing he was not frightened of. Seeing that nothing is concealed from G-d, how could He have allowed the birthright to be transferred from the older son to the younger son?
Da'at Zekenim
ויחרד יצחק חרדה גדולה עד מאד “Yitzchok was overcome by an exceedingly strong trembling;” Yitzchok actually experienced two separate fits of trembling; the first time had been when he lay bound on the altar of Moriah, expecting to have his throat slit. The second time occurred here. (B’reshit Rabbah 67,2). He asked himself what sin he had been guilty of, so that he had blessed the younger son before he had blessed the older son. He had caused Esau to groan, complaining (if I had no blessing left for him). Our author quotes a line in our version of Rashi, that did not appear in some later editions, such as the ones relied on by Luzzato.