There may be spoilers. proceed at your own risk.
Once all four Dragon Keys have been collected then you may proceed to unlock the Dragon Blade
This sword is the most ultimate weapon that you can find in the game. Not only does it increase your attack damage points by double, it also doubles your defense as well as increase your speed (when running) and jump height
A glowing halo will now outline Spike and leave a nifty fading trail (like in some Castlevania games)
Most secret passages that you haven't yet uncovered will become visible to you, and dark rooms will become lit by the ethereal glow of the blade. You would also be able to defeat certain enemies that you could not kill with just your regular attacks/weapons.
Acquiring this sword also enables the ultimate boss form as well as the best ending.
note: there will be more about this weapon in the future, on how its used as a weapon in the game as well as other things. stay tuned!