Can Darius's ability be used to pay energy costs?

Ruling: Darius's activated ability generates energy that can be used to pay any energy cost, as long as you've already cast at least one spell during that turn. He functions like an extra rune.

Sequence:

• Cast at least one spell during your turn
• Activate Darius's ability to generate energy
• Use that energy from your Rune Pool to pay for energy costs (the number in the white circle)

Nuances:

• Energy and power are resources that go into your Rune Pool and last for the whole turn, so you don't need to use them immediately
• Energy is generated by exhausting runes (or abilities like Darius), while power is generated by recycling runes
• "Pay" refers to using resources (energy or power) for costs, not the act of exhausting or recycling runes themselves