Sulfur (or brimstone, as it is sometimes also called) is one of few minerals that can burn. In burning in produces an acrid and toxic smoke that can serve as a fumigant. It played a devastating role in the Lord’s destruction of Sodom and Gomorrah (Genesis 19:24; Luke 17:29) and featured in God’s judgment warnings to Israel (Deuteronomy 29:23; Psalm 11:6; Isaiah 30:33). Mentioning “sulfur” in the present context enhances the picture of destruction as the Lord radically fumigates all wickedness.
20 And the beast was captured, and with it the false prophet who in its presence had done the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. These two were thrown alive into the lake of fire that burns with sulfur.