If Lux Crownguard dies, do you keep the +2 energy she generates?

Ruling: Yes, you keep the +2 energy even if Lux dies. The effect cannot be reacted to, and the energy goes into a conceptual "Rune Pool" as floating energy.

Nuances:

• The energy is temporary floating energy, not a rune
• The Rune Pool (where all Energy and Power goes) gets cleared at the end of every turn and at the end of every draw phase
• Even if someone tries to kill Lux in reaction to her trigger, you still get the energy because the effect cannot be reacted to