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.
In Shadowverse: Evolve, destroyed cards do not become shadows and instead go to the Cemetery. If an evolved follower is destroyed or banished, the evolved follower card is returned to the Evolve Deck face-up, while the unevolved follower goes to the Cemetery or Banished area.