Unlock True Ending:
When you have collected ALL the Dark Magic from the 20 Cursed Cacklebats you will face the Pirate Master in the Palace. If you defeat him you will be able to view the true ending.
When you complete the following tasks the corresponding wallpaper will become unlocked.
Pirate Shantae and Friends:
Complete the game with less than 100% completion in over 2 hours.
Shantae and Risky Boots:
Complete the game in less than 2 hours.
Shantae vs. ScorpGal:
Complete the game with 100% completion.
After you have completed the game you will be given the option of playing in 'Pirate' mode the next time you start another playthrough. In 'Pirate' mode you will carry over ALL of Risky's items from your previous session. Health or upgrades will be not be retained.