If I use Rhyperior V's "Drill Run" attack against a Darkrai with "Darkness Guard" Ability and only one Darkness Energy card attached, does Darkrai take 20 less damage before discarding the Darkness Energy or is the energy card discarded before the damage is reduced?

If an attack discards energy from the Defending Pokemon, the damage reduction happens first and then apply any attack effects.

Relevant cards

SWSH3_105