If I use Moonfall, do I have to pay Deflect if I move an enemy unit to that bf?

To determine if you must pay the Deflect cost when using Moonfall, you must consider how its effect interacts with the Deflect keyword.

Ruling

Yes, you must pay the Deflect cost if the enemy unit you choose to move has the Deflect keyword.

Reasoning

According to Rule 809.1.c, the Deflect keyword acts as a mandatory additional cost for any spell or ability an opponent controls that "chooses" the permanent with Deflect.

• Choosing: If Moonfall instructs you to select a specific unit to move, this selection counts as "choosing" that permanent.
• Deflect Requirement: Because Moonfall is an ability that chooses an enemy unit, if that unit has Deflect, you are required to pay the additional cost imposed by that keyword as part of the process of playing the spell.

Sequence of Play

1. Playing the Card: You play Moonfall and proceed to the "Pay Costs" step.
2. Choosing: You identify the enemy unit you intend to move.
3. Mandatory Cost: The game checks for mandatory additional costs. It identifies the Deflect keyword on the chosen enemy unit.
4. Payment:
   • If you pay the Deflect cost: You may complete the process of playing Moonfall, and the unit is moved as instructed.
   • If you cannot or choose not to pay the Deflect cost: You are prohibited from choosing that unit. If you cannot pay the cost to choose a legal target, you cannot play the ability (or you must choose not to move a unit, if the spell allows).

Summary: Deflect applies to any spell or ability that selects an enemy unit to be moved, as this selection is a form of "choosing" the permanent as defined by Rule 809.1.c.

Note: I could not fully verify this ruling against the retrieved rules, as the exact text of Moonfall was not provided.