If I play Janna, Savior at reaction speed after Atakhan attacks my battlefield, can I move Atakhan back to base to avoid killing one of my units?

Ruling: Yes. When Atakhan attacks, his "When I attack" trigger goes on the chain. If you respond by playing Janna, Savior, her ability resolves first and moves Atakhan back to his base. When Atakhan's trigger then resolves, "here" refers to Atakhan's current location (his base), where you have no units, so the effect fails to execute.

Sequence:

• Atakhan moves to your battlefield and is designated as attacker
• Atakhan's "When I attack" trigger goes on the chain
• You play Janna, Savior as a reaction
• Janna's "When you play me" trigger goes on the chain above Atakhan's trigger
• Janna's trigger resolves: you heal units and move Atakhan back to his base
• Atakhan's trigger resolves: "here" now refers to Atakhan's base where you have no units, so no unit is killed

Nuances:

• Referents like "here" are evaluated when the ability resolves, not when it triggers.