Can Reaction cards be played on their own, or only in response to another card?

Ruling: Reactions can be played as actions or base speed spells on their own. You don't need to react to something else to play them.

Nuances:

• You still need priority to play them (e.g., you can't randomly play them in the middle of your opponent's turn with nothing happening)
• They are not limited to being played only as reactions despite their name