What do the "dread and great darkness..." signify?
Rashi: They signify the Tzaros and the darkness of the exiles
This print is preferable to our print in Targum Yonasan, which cites the fourth Galus as Paras (Persia), since Paras and Madai are counted as one Galus (See the commentary "Na'ar Yonasan"). Refer to 15:12:1.2 for an alternate opinion in the Midrash. Refer also to 14:1:3:1.
According to a second opinion in the Midrash, "falling" refers to Bavel, "great" to Madai, "darkness" to Yavan, and "fright" to Edom (i.e. the terms in the Pasuk are in the opposite order of the exiles to which they refer). How can this be explained?
Maharal (Gevuros Hashem, Ch. 8, p. 51): In Nevuchadnetzar's dream alluding to the four kingdoms, Bavel is portrayed as the golden head of a statue, from where its decline was a "fall." "Great" refers to Madai, to when Achashverosh promoted Haman (Esther 3:1) to a level even greater than himself, to a godlike status (showing Amalek's great power). 1 Yavan is "darkness," for they wanted us to stop observing the Torah, which is light. "Fright" refers to Edom, due to its might.
Maharal: Achashverosh planned to thus enhance the glory of his kingdom, which contained a man elevated to such a status. This explains why Mordechai refused to bow to Haman, for his status was akin to Avodah Zarah, for in most kingdoms only the Divine is placed on a higher level than the monarch.
What is the connection between the prophecy of the exiles, and Hashem's promise to Avraham that his children would inherit Eretz Yisrael (as written above, 15:7)?
Ramban: Although Hashem promised Avraham Eretz Yisrael unconditionally, 1 that was only with regard to the initial inheritance, but remaining there permanently was conditional on his descendants' worthiness. The Pasuk is informing him here what would happen when they would sin, on four separate occasions. Following that, He informs him about the Galus that would take place imminently, due to his own sin. 2
QUESTIONS ON RASHI
Rashi writes: "Fright, darkness, etc.... allude to the misfortunes and darkness of the exiles." According to one opinion in the Midrash, "fright" refers to Bavel, "darkness" to Madai, "great [darkness]" to Yavan, "falling upon him" refers to Edom. How can this be explained?
Maharal #1 (Gevuros Hashem Ch. 8, p. 50): "Fright" represents Bavel, which received its power through Midas ha'Din. "Darkness" is Madai, for concealment is akin to darkness. Madai authorized Haman to destroy the Jewish People, and clouded their eyes due to their extended fast, which diminishes a person. "Great" refers to Yavan, the third kingdom., Yavan enjoyed great authority over the world. (Two is the smallest plural, and three is the first number that can be called numerous.) "Falling" refers to Edom, which will fall from a tremendous height (Ovadya 1:4)
Maharal #2 (ibid. p. 51): Bavel subjugated Yisrael in their bodies (Guf), they sought only authority, and are therefore termed "fright." Madai sought our lives (Nefesh) and darkened our faces. Yavan wanted to tear us away from Torah (an attack on the Sechel of Am Yisrael), and only one who is great starts up with greatness. The fourth exile, Edom, 1 includes all of these elements (Ha'Kol). 2
See Maharal (ibid. p. 48)