If Atakhan is bounced to hand or base in response to his attack trigger, does the opponent still have to kill one of their units?

Ruling: No. If Atakhan is moved to a different location or removed from the board before his attack trigger resolves, the opponent does not have to kill one of their units.

Sequence:

• Atakhan attacks and his "When I attack" trigger is placed on the chain
• Opponent responds by bouncing Atakhan (e.g., with Star-Crossed or Overzealous Fan)
• When Atakhan's trigger resolves, "here" is checked at that moment
• If Atakhan is in hand, "here" returns null and the instruction cannot execute
• If Atakhan is in base, "here" refers to base where the opponent has no units
• In either case, the ability fails to resolve

Nuances:

• Referents like "here" are checked when the ability resolves, not when it triggers
• This differs from abilities like Kha'zix's that check conditions only when triggering, not on resolution
• Once a trigger is on the chain it exists independently, but location-dependent referents are still checked on resolution