Hi, the Sulphuric Scourge whip (post-aquatic scourge) should only need an anvil and not a mythril anvil for the sake of coherency. In base Calamity, Aquatic Scourge gear can be obtained in pre-hardmode by beating it before WoF, and this is reflected in the other Corroded Fossil items only needing an anvil to craft, not a mythril anvil.