Can you use an Action spell during an existing chain, or does it have to start a new chain?

Ruling: Action spells can only start chains, they cannot be added to an existing chain. Once a chain is fully resolved (all spells on it resolve), a new chain can be started with an Action or Reaction spell.

Sequence:

• Player 1 moves a unit to battlefield, starting a showdown and gaining focus and priority
• Player 1 casts an Action spell (like Cleave), starting a chain
• Player 2 can respond with a Reaction spell (like Siphon Power)
• Both players pass priority
• Chain resolves in reverse order (Siphon Power, then Cleave)
• Player 2 receives focus and priority and can start a new chain with an Action or Reaction spell
• This process can repeat with unlimited chains until both players pass without doing anything

Nuances:

• If a player starts a chain with only a Reaction spell and passes priority, the opponent cannot respond with an Action spell - the chain ends immediately if they have no Reaction spell to play