Do I need to pay for Deflect when moving an enemy unit with Moonfall?

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

Reasoning

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

• Choosing: When you play Moonfall, you are instructed to select a specific enemy unit to move to your battlefield. Selecting this unit counts as "choosing" it.
• Deflect Requirement: Because Moonfall is an ability that chooses an enemy unit, if that unit has the Deflect keyword, you are required to pay the additional cost specified 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 cost payment steps.
2. Choosing: You identify the enemy unit you intend to move.
3. Mandatory Cost: The game checks the chosen unit for the Deflect keyword. If it has Deflect, you must pay the additional cost to proceed.
4. Payment/Finalization:
   • 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 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).

This interaction is consistent with RiftJudge FAQ #10419, which clarifies that for spells requiring you to select a unit, Deflect applies as a mandatory additional cost.