If someone moves Atakhan into an empty battlefield, when they pass focus the opponent uses Zenith Blade to move a unit into the same battlefield as Atakhan, does the attack effect "When I attack, the defender must kill one of their units here" apply to the unit or does that trigger happen the moment it moves?

The Ruling

Atakhan's "When I attack" ability activates.

While a unit does not trigger "When I attack" upon moving to an empty battlefield, the transition to combat triggered by your opponent's Zenith Blade causes your unit to gain the "Attacker" designation. At that moment, Atakhan triggers.

Sequence of Events

1. Initial Movement: You move Atakhan to an empty battlefield. Per Rule 429.1 and FAQ #8664/8689, this initiates a non-combat Showdown. Atakhan is not an attacker, and no combat designations are assigned.
2. Opponent's Response: Your opponent plays Zenith Blade at Action speed to move their unit to that same battlefield.
3. Transition to Combat: The battlefield becomes contested, and a Combat is created (per Rule 447.1).
4. Combat Designation: According to FAQ #9522, since you (or the original mover) applied the contested status first, you are the Attacker. Atakhan gains the "Attacker" designation.
5. Triggering: Atakhan's "When I attack" ability triggers because he has gained the Attacker designation for the first time during this combat. This trigger is placed on the chain (Rule 383.4.e.2).
6. Resolution: Players now have a window to play [REACTIONS] to this trigger. Once the chain resolves (LIFO), your opponent must kill one of their units at that battlefield.

Key References

• FAQ #3517: Confirms that 'When I Attack' triggers activate "anytime a unit joins a combat, including after combat has already started."
• Rule 383.4.e: Defines Attack Triggers as abilities that trigger when a unit gains the "Attacker" designation for the first time during a combat.
• FAQ #9522: Clarifies that when an opponent moves in via Zenith Blade, a combat is created where the original player (who first contested the field) is the Attacker.

Because the combat designation is granted when the units of opposing players become present at the battlefield, Atakhan's trigger is valid at that moment. The ability does not "happen" the moment he moves to the empty battlefield; it waits until the combat condition is met by your opponent's entry.

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