From Bulbapedia, the community-driven Pokémon encyclopedia.
The Key System menu
The Key System (Japanese: キーシステム Key System) is a feature of the Unova Link in Pokémon Black 2 and White 2. It allows players to change the difficulty level of the game, switch between Black City and White Forest, and change the Mystery Door in the Underground Ruins. They can be shared between players using the Unova Link, but players can only share the keys that are exclusive to their version.
Difficulty
For the first time in a Pokémon game, there are different difficulty levels. There are three different modes: Easy Mode (Japanese: アシストモード Assist Mode), Normal Mode (Japanese: ノーマルモード Normal Mode), and Challenge Mode (Japanese: チャレンジモード Challenge Mode). Only Normal Mode is unlocked by default; Easy Mode and Challenge Mode must be unlocked through the course of the games. Because they are unlocked only after defeating the Champion, the player must start a new game in order to play the main storyline with a different difficulty.
Easy Mode decreases the opponents' artificial intelligence and the level of their Pokémon. As such, battles become easier. The player will be rewarded with the Easy Key (Japanese: アシストキー Assist Key) after defeating the Champion in Pokémon White 2.
Challenge Mode increases some opponents' artificial intelligence and the level of their Pokémon, starting from 1 level and reaching 5 levels by the end of the game. Gym Leaders and Elite Four members will also have an extra Pokémon on their team, as well as different movesets and held items. Their IVs are also increased to 30. The player will be rewarded with the Challenge Key (Japanese: チャレンジキー Challenge Key) after defeating the Champion in Pokémon Black 2.
To unlock Easy Mode in Black 2 and Challenge Mode in White 2, the respective keys for each version need to be transferred from a player with the opposite version via the Unova Link.
City
The version exclusive cities, Black City and White Forest, are found by default in Black Version 2 and White Version 2, respectively. They can be unlocked in the opposite game if certain conditions are met.
After completing the five areas of Black Tower in Black 2, the player will be rewarded with the Tower Key (Japanese: まてんろうのカギ Skyscraper Key). This can be transferred to White 2, which will unlock access to Black City. Likewise, after White 2 players have completed five areas of White Treehollow, the Treehollow Key (Japanese: じゅどうのカギ Tree Hollow Key) can be transferred to Black 2. This unlocks White Forest for Black 2 players.
Players of either version can use the Unova Link to switch between cities once both are unlocked. However, the cities cannot be switched if the player had saved their game within the current city.
Mystery Door
A mysterious door in Clay Tunnel gives access to the Underground Ruins, where the legendary titans can be found, though only one titan will appear at a time, depending on which key is active. Regirock is the default Pokémon in both Black 2 and White 2. Upon catching Regirock, and not just defeating it or obtaining it from another game, Black 2 players are rewarded with the Iron Key (Japanese: くろがねのカギ Iron Key) and White 2 players with the Iceberg Key (Japanese: ひょうざんのカギ Iceberg Key); these allow access to Registeel and Regice, respectively.
While Registeel and Regice are exclusive to Black 2 and White 2 respectively, players of the opposite version games can obtain their final legendary titan by transferring the appropriate key through the Unova Link.
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