Can I activate Lux Crownguard's ability to add 2 energy in response to a kill spell (like Vengeance), then use that energy to play Retreat to save Lux?

Ruling: Yes, you can activate Lux's ability to add 2 energy in response to Vengeance, then use that energy to play Retreat. However, the more standard play is to react with Retreat directly and use Lux's ability to pay the cost while finalizing Retreat on the chain, since you don't need to add resources before playing a spell.

Sequence:

• Opponent plays Vengeance targeting Lux
• You receive priority to react
• Option 1: Activate Lux to add 2 energy, then play Retreat using that energy
• Option 2 (more standard): Play Retreat as a reaction, then activate Lux's ability to pay the cost as part of the Play action

Nuances:

• Abilities that add resources (with the "Add" tag) resolve immediately without a chain and can be activated during the process of paying for a spell's cost
• You can activate abilities with the "Add" tag as part of finalizing a spell or ability on the chain