How does priority passing work when starting a chain via casting a spell or combat? Can the active player play multiple reaction spells before the opponent gets priority, or does priority alternate after each spell?

Ruling: When you play a spell, you can hold priority and play multiple reaction spells before passing priority to your opponent. Priority then passes back and forth (you as long as you want -> them as long as they want -> you as long as you want, etc.).

Sequence:

• Active player plays a spell and can hold priority to play additional reactions
• When active player passes, opponent gets priority and can play reactions as long as they want
• Priority continues alternating until both players pass in sequence
• After both pass, the top item of the chain resolves
• After each item resolves, the controller of the most recent remaining item (the one coming up next) gets priority first
• Players can add to the chain again, and this continues until the chain is empty

Nuances:

• You cannot "chain block" - counterspells can target any object on the chain regardless of what was played after
• The chain does not resolve in its entirety once both players pass; it resolves one item at a time with priority passing after each resolution
• Permanents (like Shen) resolve immediately when played and cannot be reacted to, but players get priority again after they resolve
• Triggered abilities that meet timing are added to the same chain before priority is passed