"As soon as you reach an ash covered area after the stairs, an Ashen Idol will emerge. It will blow up and cause several fire pillars to erupt around it. So once you see it surface, retreat to a safe distance and let it throw its flame tantrum before approaching it and using a Smelter Wedge to destroy it"
Do not destroy this 1st Ashen Idol until after you have killed the Fume Knight, otherwise you will run out of Smelter Wedges.