fantasysmp_fabric/src/main/resources/witherite_axe.json

{
  "type": "minecraft:smithing_transform",
  "template": "minecraft:netherite_upgrade_smithing_template",
  "base": "minecraft:netherite_axe",
  "addition": "minecraft:wither_skeleton_skull",
  "result": {
    "id": "fantasysmp:witherite_axe"
  }
}