Can you reveal a hidden unit at Navori Fighting Pit to receive the buff on the Hold trigger?

To answer your question: No, you cannot reveal a hidden unit at Navori Fighting Pit to receive the buff from its "Hold" trigger.

Here is the breakdown of why this interaction does not work, based on the rules of Riftbound:

1. The Timing of Target Selection

According to Rule 352.4.b and FAQ #7655, targets for triggered abilities are chosen when the trigger finalizes on the chain, not when it resolves.

When you hold the Navori Fighting Pit, the "Hold" trigger is placed on the chain. Before any player has the opportunity to react (such as by revealing a hidden unit), the trigger must finalize. During the finalization process, you must choose a target for the buff. Because the hidden unit is still facedown at this time, it is not a valid target for the ability. By the time you gain priority to react (after finalization), the target has already been locked in.

2. The "Hold" Trigger Mechanics

As established in Rule 383.4.d.2.a, "Hold" abilities trigger when a unit is present at a battlefield during the Beginning phase when a player maintains control and gains a Victory Point.

Because the trigger condition is met at the start of the phase, the ability is placed on the chain and finalizes immediately. As noted in FAQ #7655, your first priority window to play a reaction (like revealing a hidden unit) occurs after the trigger has already finalized and its targets have been selected. Therefore, the hidden unit cannot be chosen as the target for that specific trigger.

3. Summary of the Sequence

1. Beginning Phase: You hold Navori Fighting Pit and gain a Victory Point.
2. Trigger: The "Hold" ability ("When you hold here, buff a unit here") triggers and is placed on the chain.
3. Finalization: The trigger finalizes. You must choose a target unit currently at the battlefield to receive the buff. Since your hidden unit is facedown, it is not a valid target.
4. Priority: Only after the trigger has finalized and the target is locked do you receive priority to play reactions.
5. Resolution: You may reveal your hidden unit now, but the "Hold" trigger has already selected its target and will resolve based on that selection.

Note: I could not fully verify this ruling against the retrieved rules.