When Red Darius is played and his ability activates (either as the first card or second card of the turn, or when readied by another card), does this open a reaction opportunity for the non-turn player?

Ruling: Yes, Darius' triggered ability opens a chain allowing for reactions to be played in both scenarios (when played as the first card then readied, or when played as the second card with his ability activating on play).

Nuances:

• Draven does not have a triggered ability and does not create a chain
• Vayne has a triggered ability and creates a chain