Chagigah 5b: They are angels. Radak - Er'el is the name of the angel. Chazal said, Er'elem defeated the Metzukim (Kesuvos 104a; angels wanted the Neshamah of Rebbi, against the people praying for him to live). Just like the name Mal'ach refers to a physical messenger and not a spiritual messenger, so the word Eri'el.

Rashi: It is the Mizbe'ach.

Targum Yonasan: I will appear to them. Radak - he explains this like two words, Era'eh Lahem.

Malbim: They are the Giborim of Machaneh Ashur.

Refer to 22:12:1:1 and the note there.

Rashi: The Navi prophesized consolations, and says that the punishments already consumed them, and from now and onwards Hashem will rise to redeem them. For the Mizbe'ach, they already screamed and eulogized outside in the streets with crying and lamenting.

Radak: This discusses the punishment that will come before the fall of Machaneh Ashur. Their angel, that they usually send it from city to city, and initially they were messengers of Shalom, now they scream outside, for the enemy came to the land; they cry bitterly.

Malbim: Ashur's Giborim screamed outside Yerushalayim - Ravshake and his men spoke in Hebrew to revile the living G-d. Also refer to 33:7:3:3.

Rashi: They are angels that usually, their mission is to give tidings of Shalom. Now, they say "Nashamu Mesilos?" (8).

Radak: The matter is repeated in different words.

Malbim: This refers to Chizkiyah's messengers to Sancheriv to make Shalom with him. They had bad tidings (verse 8).