Shadows are what destroyed cards, discarded cards and played Spells become. Whenever a card on the field is destroyed, a card is discarded or a Spell card is played, it becomes a shadow and is added to its player's graveyard.
Shadows can be used for Necromancy effects or other cards that require a certain number of shadows.
Cards that are banished, transformed or fused do not become shadows.
Certain cards can add to the number of shadows a player has.