Unlockable

From Styleguide

unlockable

Hidden content or items that are unlocked through specific in-game actions. Unlockables are usually not essential to play and/or complete the game, but are added as a bonus for players who complete difficult tasks.

Example: GoldenEye 007's Invisibility mode is one of the most difficult- to-obtain unlockables in videogame history.

Also see: Achievement, bonus, cheat code, Easter egg.