When a creature with a 'when attacking' trigger enters the battlefield and that trigger goes on the stack, can the opponent play an Action when they receive priority, or are they limited to Reactions since a chain has started?

Ruling: Once a chain has started (such as when a 'when attacking' trigger goes on the stack), players can only play Reactions, not Actions.

Nuances:

• Actions cannot be added to an existing chain
• The 'when attacking' trigger starting the chain is what restricts players to only Reactions