What is the essence of the Nevu'ah for Damesek?
Radak: Melech Ashur will destroy it; he destroyed Shomron (Malchus Efrayim) afterwards. The latter is mentioned in this Nevu'ah, for Retzin Melech Aram and Pekach ben Remalyahu Melech Yisrael joined to fight against Yerushalayim. Radak - it mentions Damesek, for it was the capitol - "Rosh Aram Damesek" (7:8).
What is "Musar me'Ir"?
Rashi: It will be removed from being a city.
Malbim: It will no longer be considered a city, rather, an Iy (ruin) via houses collapsing.
What is "Me'i [Mapalah]"?
Rashi: It is an expression of lowliness and depth.
Radak: It will be lowered, and become a rock pile.
Malbim: Refer to 17:1:2:2.
