What is "Mispad Hadadrimon v'Vik'as Megidon"?
Rashi citing Megilah 3a, based on the Targum 1 : These are the eulogies of Achav (who was killed by Hadadrimon 2 ) and Yoshiyahu (Pharaoh Necho killed him in the valley of Megidon).
Radak: This eulogy was known to them. We do not find a hint to it in Tanach.
3a: Without the Targum, we would not understand this (we do not find that he was eulogized there).
Malbim: Ben Hadad, who served the idolatry Rimon, killed him. Yoshiyah's eulogy was for Yachid ? Yisrael had only one king then. Yehudah and Binyamin eulogized him. At the time of Achav, there were two kings. It is considered a eulogy for a Bechor, since he was king over 10 tribes; the 10 eulogized him. The eulogy of Mashi'ach ben Yosef is compared to both of them, for all tribes will eulogize him.