What is the story behind mourning for Hadad-rimmon in Zechariah 12:11?

Zechariah, like Revelation which draws on it, is a tough nut to crack but very rich. In the twelfth chapter, the prophet records the word of Yahweh that

   The sorrow and mourning in Jerusalem on that day will be like the great mourning for Hadad-rimmon in the valley of Megiddo. —12:11 (NLT)

Is it known historically who Hadad-rimmon was, what led to his demise, and how he was mourned? Is there apocalyptic significance to the reference to Megiddo? Was this an event well-known in the ancient world?

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like the mourning of Hadadrimmon in the Valley of Megiddon: Hadadrimmon has no connection to the Valley of Megiddon. These are, rather, two cases of mourning. [The first is] like the mourning of Ahab the son of Omri, who was slain by Hadadrimmon the son of Tabrimmon in Ramoth Gilead, as it is stated (I Kings 22:36): “A cry passed through the camp.” That is the mourning [of Ahab. The second case is] like the mourning of Josiah the son of Amon, who was slain by Pharoah the lame in the Valley of Megiddon, as it is stated (II Chron. 35:25): “And Jeremiah lamented for Josiah, and all the singing men and singing women spoke in their laments, etc.”

-Rashi

Implicit in Rashi's interpretation is that even though there were two events, the author treats them as one. The Hebrew hermeneutic treats parallel accounts like overlays of one 'event'.