Shadow

Shadow is a keyword ability on creatures that serves as both an evasion ability and a blocking restriction. An attacking creature with shadow can only be blocked by a defending creature with shadow, and a defending creature with shadow can only block an attacking creature with shadow. It was first seen in the Tempest block, and was revisited in the Time Spiral block. Shadow is found on black, white or blue creatures, while a few red and green walls are able to block shadow creatures.

From the Comprehensive Rules

• 502.8. Shadow
  • 502.8a Shadow is an evasion ability.
  • 502.8b A creature with shadow can’t be blocked by creatures without shadow, and a creature without shadow can’t be blocked by creatures with shadow. (See rule 309, “Declare Blockers Step.”)
  • 502.8c Multiple instances of shadow on the same creature are redundant.