The chapter ends with a short “burden” for the Philistines. This one is dated to the year king Ahaz died, or around 720 BC (Ludlow 1982, p. 189.)
Textual: Even though these verses are a burden for the Philistines, it is dependent upon the previous burden of the Assyrians (“the rod of him that smote thee” refers to the removal of the political/military power of Assyria). The destruction of the Assyrians is the reason that the Philistines might be joyful, because Assyria has also oppressed them. Nevertheless, the Lord tells them that they should not rejoice in the destruction of Assyria, because that will not spell salvation for them.
Translation: The KJV renders the word “seraph” as “fiery flying serpent.” Seraph is “the burning one” and is a term that is used to describe the feeling of burning associated with the venom of a serpent. Thus the “seraph” is necessarily a poisonous snake. This is the same term used to describe the venomous serpent episode during the Exodus (Numbers 21:7). The “flying” part may simply indicate the speed of the attack of the serpent.
Literary: This verse continues Isaiah’s use of agricultural terminologies for descendants. Once again we have a “root” and a “fruit.” The “root” refers to the lineage of the fallen Assyria, either as a nation or as a symbol. The “fruit” is what will develop out of that line. The prophecy says that Palestine should not rejoice in the destruction of Assyria, because from that same root stock (of evil intent, if not the nation) will come another force to strike against Palestine, and that descendant of the “snake” of Assyria will be even worse (“fiery flying”).
Historical: The Philistines were exulting over the defeat of Shalmaneser V (727-722 BC) or perhaps the freedom gained after Sargon’s conquest of Samaria and Israel (722-721 BC). While these historical “snakes” were dead, one of their descendants, would prove to be worse. Sennacherib conquered Philistia in 701 BC. (Ludlow, 1982, p. 190 and 316.)