Can you play a reaction spell at the end of an opponent's turn (like in Magic: The Gathering)?

Ruling: No, you cannot play a reaction spell at the end of an opponent's turn without something to react to. You need something to initiate a chain or showdown in order to react during your opponent's turn.

Nuances:

• If a vanilla permanent (no effects) is played, you cannot respond with a reaction because no effect is on the stack
• When a permanent is cast, it goes on the chain and leaves immediately, creating a brief window to react
• Once a permanent enters the battlefield (ETB), there is no chain unless it triggered an ETB effect