A silhouette is a shadow shape that is created by backlighting (the light comes from behind the figures). Gray or black silhouettes without any detail look like figures from a shadow play.
Exercise 1: cut out photos of figures from magazines (for example people, animals) or other sources. Place the cut-out figures on paper and draw the outline as lightly as possible. Make shadow shapes by making them completely gray or black, without any detail and without a clearly visible outline. If you want them to be completely pitch black you should use a marker or (pen/brush and) Indian ink.
Exercise 2: Shadows can be mysterious and even creepy. This quality is often used in movies. In the following film shot from an old horror movie, you see the shadow shape on the wall of a creepy figure walking up the staircase to a door. Using a soft pencil (2B or larger), draw this entire shot on a small format (e.g. 15 x 10 or 12 x 8 cm.) and possibly think of another creepy silhouette walking up the stairs. Pay attention to the differences in gray tones. Try not to draw hard contour lines around the shadow shapes.
EXTRA: Invent your own small shadow play.
In the following drawing by a Lithuanian artist, a small figure on the left is being carried along by two spooky figures, of which the one behind has a skull. In the background you can see the large shadow shape/ghost of death with his large scythe. All shadow effects are created by hatching with pen and Indian ink.