If I have a hidden Wages of Pain card at a battlefield, and my opponent moves in to conquer the other battlefield on their turn that has one of my units, can I react by revealing the hidden card at my other battlefield to Wages their unit?

Yes, you can react by revealing your hidden Wages of Pain at your other battlefield to target the opponent's unit, provided you meet the timing and targeting requirements.

The Ruling

According to RiftJudge FAQ #6275 and #1525, you can play hidden cards as a reaction from any battlefield, even if the showdown or event is occurring at a different battlefield.

However, there is a critical targeting restriction: Hidden cards cannot target units at a different battlefield than where they are hidden (Rule 811.1.d.2 and FAQ #7786).

How it works in your scenario:

1. The Trigger: Your opponent moves a unit to a battlefield you control, causing a showdown. This event (the move/showdown) allows you to play a reaction.
2. The Reaction: You have priority to play a reaction. You choose to reveal your hidden Wages of Pain at your other battlefield.
3. The Targeting Constraint: Because Wages of Pain requires you to "Deal 3 to a unit at a battlefield," you must choose a target at the same battlefield where the card is hidden.
   • If the opponent's unit is at the battlefield where you hid the card: You can target it.
   • If the opponent's unit is at the battlefield where the showdown is happening (and that is different from where the card is hidden): You cannot target that unit with the hidden card.

Summary Checklist

• Can you react? Yes, the showdown at one battlefield gives you the opportunity to play a reaction from anywhere.
• Can you target the unit? Only if that unit is at the same battlefield where you hid the Wages of Pain. If the unit is at a different battlefield, you cannot target it with that specific hidden card.
• Timing: Ensure the card was hidden on a previous turn, as you cannot play a hidden card on the same turn it was hidden (FAQ #4293).

This is my interpretation based on Rule 811.1.d.2 and the cited FAQs. No official FAQ exists for this specific interaction with Wages of Pain, but the targeting restriction for hidden cards is a core mechanic.