Does Diana Lunari effect still go off if you pay 1 prior to the unit being gusted in response?

To determine if Diana, Lunari’s ability still goes off if the unit is "gusted" (recalled or removed) in response to the ability, we must look at how triggered abilities with optional costs and "if you do" instructions resolve.

Ruling

Yes, Diana, Lunari's effect still goes off even if Diana is removed from the battlefield in response to the ability, provided the ability successfully resolves.

According to the Riftbound rules and game mechanics:

1. Triggering: When a showdown begins at Diana's battlefield, the triggered ability is placed on the chain.
2. Optional Cost: The cost [1] is an optional component within the ability's instructions, formatted as a "do X to do Y" structure.
3. Payment Timing: Per the logic established in FAQ #1951, costs within instructions for triggered abilities are paid on resolution, not when the ability is placed on the chain.
4. Effect Resolution: As established in FAQ #6408, once an ability is on the chain, it resolves independently of its source. Even if Diana is removed from the board after the ability is on the chain, the ability remains on the chain and will resolve.

Sequence of Events

1. Trigger: A showdown begins. Diana’s ability triggers and is placed on the chain.
2. Response: The opponent plays a spell or ability to remove (e.g., "gust") Diana from the battlefield.
3. Resolution: The removal resolves, and Diana leaves the board.
4. Ability Resolution: The triggered ability is still on the chain. When it resolves:
   • The controller is given the opportunity to pay [1].
   • If the controller pays [1], the "If you do" condition is met, and the effect ([Predict], reveal, and potentially draw) occurs.
   • Because the ability does not require Diana to be present at the time of resolution to function (the card text does not specify "if I am here" or similar restrictions), the effect executes fully.

Note: This is my interpretation based on the mechanics of triggered abilities and resolution independently of the source. While FAQ #6408 confirms that triggered abilities resolve regardless of the source's presence, no official FAQ exists specifically for the interaction of Diana's specific "If you do" payment sequence when the unit is removed.