If I have a Draven, Vanquisher on the battlefield with a hidden Fight or Flight, and my opponent moves his Vex to combat with the Draven, on Draven's defender trigger, can I respond with Fight or Flight for its hidden cost or does Vex tax the hidden card?

Yes, you can respond with your hidden Fight or Flight, but you must pay the additional cost imposed by Vex.

Ruling

According to the official rulings for Vex and Hidden cards:

• Hidden sets the base cost of the spell to 0.
• Vex's ability acts as a cost increase for enemy spells, which is applied after the base cost is set to 0.
• Therefore, you must pay the additional cost imposed by Vex (1 Energy + 1 Power) to activate the spell from Hidden.

Sequence of Events

1. Combat Initiation: Your opponent moves Vex to the battlefield where Draven, Vanquisher is located, triggering combat.
2. Triggered Abilities: Draven's "When I attack or defend" ability triggers and is placed on the chain.
3. Reaction Window: A reaction window opens. You may now play your hidden Fight or Flight.
4. Cost Calculation:
   • Hidden reduces the base cost of Fight or Flight to 0.
   • Vex's passive ability adds an additional cost of 1 Energy + 1 Power to enemy spells.
   • The final cost to play your hidden spell is 1 Energy + 1 Power.
5. Resolution: Once you pay the cost, Fight or Flight is added to the chain. The chain resolves in LIFO (Last-In, First-Out) order:
   • Fight or Flight resolves first, moving the target unit to its base.
   • Draven's triggered ability then resolves. As per the FAQ, you decide whether to pay the 1 Fury Power cost at the moment of resolution to gain the +2 Might buff.