Can Azir be played from hand during a showdown and immediately use his Ascendant Action ability, similar to how Shen/Rengar can use their Reaction abilities when played?

Ruling: No, Azir must already be on the board to use his Ascendant Action ability. Unlike Shen/Rengar who have Reaction timing (which allows use when played from hand), only Azir's ability has Action timing, not Azir himself.

Nuances:

• Shen/Rengar work differently because they have Reaction timing on the card itself, allowing their abilities to be used when played from hand during appropriate windows
• Azir needs to be on the board (not specifically on the battlefield) before his Action ability can be activated