Shadow is an evasion ability defined in CR 702.28. The core rule (702.28b) states that a creature with shadow can only block creatures with shadow, and a creature with shadow can only be blocked by creatures with shadow. This creates a parallel 'plane' of combat that shadowless creatures cannot interact with.
This restriction works in both directions: a creature with shadow cannot block a creature without shadow, and a creature without shadow cannot block a creature with shadow. Neither side can bridge the gap unless another effect overrides normal blocking restrictions.
If two creatures both have shadow, they interact with each other normally — they can freely block and be blocked by one another, just as if shadow didn't exist between them.
Example: Your opponent attacks with a 2/2 Dauthi Slayer (which has shadow). You have a 5/5 Craw Wurm (no shadow) and a 1/1 Dauthi Mercenary (shadow). Only the Dauthi Mercenary can legally block the Dauthi Slayer; the Craw Wurm cannot, despite being much larger.
Note that effects which say a creature 'can block as though it had shadow' or 'can't be blocked except by creatures with shadow' interact with these rules as explicit overrides, but absent such effects, the shadow-vs-no-shadow barrier is absolute (CR 702.28b, 509.1b).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.