If you play Falling Star and kill a unit, can you use Darius' Legend ability to react to your Falling Star, and use that 1 Energy to pay for Immortal Phoenix to play it from the Trash?

Ruling: Yes, this works. Falling Star resolves before its reflexive triggers, so the Legion condition is met when you cast Darius' ability. Reactions that generate resources can be cast as part of paying a cost and can't be responded to, so once the damage kills a unit and Immortal Phoenix triggers, you can pay the cost with Darius (you also need to pay red power). The triggered abilities from Falling Star count as Falling Star's damage (spell damage).

Sequence:

• Play and resolve Falling Star (Legion condition now met)
• Falling Star's reflexive triggers go on the stack
• Reflexive trigger deals damage and kills a unit
• Immortal Phoenix triggers
• Use Darius' Legend ability as part of paying Immortal Phoenix's cost
• Pay remaining costs (red power) and play Immortal Phoenix from Trash

Nuances:

• The reflexive triggers from Falling Star count as both triggered ability damage AND spell damage (from the original Falling Star source)
• Falling Star must fully resolve before its reflexive triggers occur, which is why the Legion condition is satisfied in time