Can Hidden Blade be used as a reaction when set as a hidden card, and can Sett's ability trigger mid-chain to bounce a unit and cause the opponent to still draw cards from Hidden Blade even though the unit didn't die?

Ruling: Hidden cards gain "reaction" when you've paid their hidden cost, so Hidden Blade can be used as a reaction during a chain. Sett's ability can trigger in the middle of a chain when a unit would die. The opponent still draws 2 cards from Hidden Blade even if the targeted unit is bounced before resolution, because Hidden Blade's effects are separate and not conditional on each other.

Sequence:

• Hidden Blade is played as a reaction from hidden during a chain
• Hidden Blade targets a unit
• Sett's ability triggers before the unit dies, bouncing it to base
• The targeted unit is no longer at the battlefield
• The opponent (Hidden Blade's controller) still draws 2 cards because the kill effect and draw effect are separate

Nuances:

• The controller who draws cards is determined at Hidden Blade's resolution, not when it's played
• If control of the unit changes before resolution, the new controller draws the cards
• If the unit leaves the board entirely (bounced, banished, recycled, killed by other means) before resolution, nobody draws cards because the controller can no longer be determined
• The controller is not a target according to targeting rules