How does Darius Legend's ability work, particularly regarding when it can be activated and whether it can be used on the opponent's turn?

Ruling: Darius Legend can generate 1 energy by tapping himself, but only if you have already played another card that turn (Legion requirement). This works on both your turn and your opponent's turn - during your opponent's turn, you can play Action or Reaction cards which then allows you to use Darius's Legion ability as a Reaction to generate energy.

Sequence:

• Play a card (any card that turn)
• Tap Darius to generate 1 energy
• The energy must be spent that turn or it's lost

Nuances:

• Darius doesn't actually un-exhaust a rune; the energy generated doesn't carry over to the opponent's turn if unused
• Spells and abilities that add resources (like Darius) resolve immediately and can't be reacted to
• Using Darius mid-chain doesn't end the chain - other items on the chain can still be reacted to