Can a player use Stupefy to react to Mindsplitter's ability that reveals and discards from opponent's hand?

Ruling: Yes, Stupefy can be played in response to Mindsplitter's triggered ability. You cannot react to units themselves in Riftbound, but you can react to triggered abilities.

Sequence:

• Mindsplitter resolves and enters play
• Mindsplitter's WYPM (When You Play Me) triggered ability goes onto the chain
• Priority passes to both players before the ability resolves
• Player B can play Stupefy at this point to counter the triggered ability

Nuances:

• The distinction is important: players react to the triggered ability, not to the unit card itself