The Silver Key is a magic item of tremendous power.  It is actually made of Mithril  and is covered in beautiful arabesques.   
 
If it is put under the pillow when one goes to sleep,  the sleeper has a the most refreshing sleep and has lucid dreams.

Further more it is also known to unlock doors at "soft places"  that can lead one bodily into the DreamWorld.

When in the dream world, either in dream state or bodily,   the dreamer can dreamwalk as per the power and can lucid dream ( control the dreamscape) with near perfect results.

It is rumored to be able to do other things, but there are so many stories, mostly untrue about this key so who knows if this is the limit of the power of this artifact.