Custom items.
A challenge declares items of its own: a base item with a name, lore, enchantments, a rarity, whether it breaks, a glint. They are given, checked, and used like any item, and a step reads whether a player holds one.
Using them.
Using an item, eating or drinking, dropping, picking up, smelting, and enchanting are events. A custom item's event is how a wand works: when the item is used, do the magic.
Inventories and drops.
Steps give, take, and equip items, clear, copy, and swap inventories, lock slots, and set drops: what a block or a mob drops, multiplied, replaced or cleared.