The Assyrian king uses a calculated campaign of psychological warfare to break the spirit of his enemies. By boasting of a long line of past victories, he attempts to instill deep fear and hopelessness. He asks a sharp rhetorical question about the whereabouts of the former rulers [ביאור שטיינזלץ]. This challenge operates on two levels. It mocks the gods of the conquered nations who failed to protect their worshippers, and it highlights the total failure of the human kings who tried to resist the unstoppable Assyrian war machine [רד״ק]. The underlying message is absolute: every one of these leaders and their kingdoms faced total destruction, death, or exile.
The boastful claims single out Sepharvaim as a major city to highlight that it was an exceptionally large, important, and powerful metropolis, far greater than the other conquered territories [מצודת דוד, רד״ק]. The destruction of other kingdoms, such as Hena and Ivvah, further adds to the overwhelming record of defeated rulers [רש״י, ביאור שטיינזלץ].
The ultimate purpose of reciting this long, grim list of fallen empires is to deliver an unmistakable warning. The listener is meant to realize that they are simply the next target on the Assyrian conquest list. It is a promise that, sooner or later, they will share the exact same tragic fate as the kings who came before them [ביאור שטיינזלץ].