Can actions and reactions be activated at the end of an opponent's turn in Riftbound, or are they tied specifically to showdowns on opponent's turns?

Ruling: Phase changes do not grant priority windows in Riftbound. You cannot play actions or reactions at the end of an opponent's turn unless there is a trigger (such as an end-of-turn effect) that goes on the chain.

Sequence:

• Phase changes happen without priority windows
• If an end-of-turn trigger occurs, it goes on the chain
• Players can then react to that trigger

Nuances:

• Everything in Riftbound is base speed (similar to sorcery speed in MTG) unless explicitly tagged otherwise
• Only cards specifically tagged as "Reaction" can be used during reaction windows
• Activated abilities on permanents (like Lee Sin's buff ability) are base speed unless tagged as reactions
• The term "action" only applies to things that specifically say "action" - most things are simply base speed