Think of it like needing a special key to potentially make certain rare Pokémon appear in your world. Some Pokémon, especially legendary or mythical ones, won't spawn unless you have obtained a specific Key Item first.

- Check Your Bag: The game checks if you are carrying the required Key Item in your main inventory. - Check Storage (Maybe): Depending on the server's configuration, specifically the inventory_check_shulker_boxes and inventory_check_bundles settings, the game might also look inside any Shulker Boxes or Bundles you have in your inventory. - Unlock the Spawn: If the game finds the required Key Item on you, this condition is met! This means the Pokémon that requires this item is now added to the list of Pokémon that could potentially spawn near you, provided all other conditions (like being in the right biome or time of day) are also met.

* To have a chance for Articuno to spawn naturally, the game checks if you have the Tidal Bell (mythsandlegends:tidal_bell). If you have it (and you're in a cold biome, etc.), Articuno might appear! If you don't have the Tidal Bell, Articuno won't spawn naturally based on this rule.

* Not Used Up: Usually, Key Items are not consumed or taken away from you when the Pokémon spawns naturally. The default configuration setting ignore_key_items = true typically prevents this. * Force Spawning: Having the Key Item might also be necessary if you want to use mechanics like “force spawning” (often by right-clicking the Key Item, if enabled via the enable_force_spawning setting) to try and summon the Pokémon directly.

In short: Get the right Key Item → Carry it with you → Unlock the chance for specific rare Pokémon to spawn.