In Isaiah 7:4, what did God call the two kings threatening Ahaz, telling him not to fear?

Answer: Two smoldering stubs of firewood

Ahaz was told to be calm and not fear "these two smoldering stubs of firewood" (KJV: "smoking firebrands"), Rezin and Remaliah's son.

“and say to him: Calm down and be quiet. Do not be afraid or disheartened over these two smoldering stubs of firewood—over the fierce anger of Rezin and Aram and of the son of Remaliah.”— Isaiah 7:4 (BSB)