Can an action card be played in response to another action card, or can only reaction cards be chained on top of action cards?

Ruling: Only reaction cards can be played in response to action cards. Action cards cannot be played in response to other action cards.

Sequence:

• When an action card is played, opponents can only respond with reaction cards
• Action cards can only be played when a player has priority during normal timing
• During a showdown, only cards with the keyword "action" or "reaction" can be played

Nuances:

• Hidden cards gain the keyword "reaction" while hidden, so a hidden card like Hidden Blade could be played in response if it was already hidden
• Hidden Blade cannot be cast as an action from hand while another card (like Disintegrate) is trying to resolve