If I charm my opponent's unit Hwei in base with a charm to move to battlefield X, but then my opponent who controls battlefield Y reacts with a hidden Tideturner in battlefield Y to swap with Hwei, what happens?

This interaction involves resolving a chain where Charm is moving your opponent's unit, Hwei, and your opponent is responding by playing a Tideturner from hidden to swap their positions.

Here is the step-by-step resolution based on the game rules and established FAQs:

1. The Chain Setup

• Item 1: You play Charm, targeting Hwei in the base to move to Battlefield X.
• Item 2: Your opponent plays Tideturner from "Hidden" at Battlefield Y in response. Tideturner enters play at Battlefield Y, placing its "When you play me" triggered ability on the chain.

The chain currently looks like this (from bottom to top):

1. Charm (Targets Hwei at Base)
2. Tideturner's ability (Targets Hwei at Base)

2. Resolution (LIFO - Last In, First Out)

The chain resolves in reverse order:

1. Tideturner's ability resolves: You chose Hwei at the base as the target for Tideturner's swap ability. Tideturner moves from Battlefield Y to the base, and Hwei moves from the base to Battlefield Y.
2. Charm resolves: Charm's instruction is to "Move an enemy unit" (targeting the original Hwei). Because Hwei moved to Battlefield Y due to the Tideturner resolution, and Charm was targeting that specific game object, the targeting relationship is maintained (per the Alpha Strike/Flash FAQ precedent regarding targeting relationships being maintained as long as the object stays on the board).
   • However, Charm's instruction is to move the unit to Battlefield X. Because Hwei is now at Battlefield Y, Charm will successfully complete its instruction and move Hwei from Battlefield Y to Battlefield X.

Summary of the Outcome

• Tideturner ends up at the Base.
• Hwei ends up at Battlefield X (having been moved there by the resolution of the Charm).

Important Note on Targeting: According to RiftJudge FAQ #11202 and Core Rule 359.3.e.3, a spell maintains a targeting relationship with a game object as long as that object remains on the board, even if it moves to a different location. Since Tideturner's ability is a specific exception that allows it to target units anywhere (per FAQ 811.1.d.2 and FAQ 6517), the swap is legal. When the subsequent Charm resolves, it still "remembers" Hwei as its target and completes its move instruction, relocating Hwei from the location it was swapped into (Battlefield Y) to the destination specified by the spell (Battlefield X).

This is my interpretation based on the interaction of targeting and resolution rules. No official FAQ exists for this specific chain of events involving Charm and Tideturner.