The opponent's Black Market prism star stadium card is in play - https://assets.pokemon.com/assets/cms2/img/cards/web/SM9/SM9_EN_134.png
The text of the Black Market card reads: "When a Darkness Pokémon (yours or your opponent's) that has any Darkness Energy attached to it is Knocked Out by damage from an opponent's attack, that player takes 1 fewer Prize card."
The opponent's active Pokemon is a Darkness type with 10 HP remaining, with Darkness energy attached.
The attacking Active Pokemon is a White Kyurem (Lost Thunder 63/214) - https://assets.pokemon.com/assets/cms2/img/cards/web/SM8/SM8_EN_63.png
The attack "Field Crush" is used, dealing 30 damage to the opponent's active Pokemon (which knocks it out), and also causes the effect "If your opponent has a Stadium card in play, discard it".
It is clear that the opponent's Pokemon is knocked out before the stadium card is discarded (to the Lost Zone).
What is unclear is whether the effect of the stadium card still applies to any prize card(s) the attacking player draws for the knockout, or if the effect of the attack removes the stadium card from play before the prize card(s) are drawn.
The text of the Black Market card reads: "When a Darkness Pokémon (yours or your opponent's) that has any Darkness Energy attached to it is Knocked Out by damage from an opponent's attack, that player takes 1 fewer Prize card."
The opponent's active Pokemon is a Darkness type with 10 HP remaining, with Darkness energy attached.
The attacking Active Pokemon is a White Kyurem (Lost Thunder 63/214) - https://assets.pokemon.com/assets/cms2/img/cards/web/SM8/SM8_EN_63.png
The attack "Field Crush" is used, dealing 30 damage to the opponent's active Pokemon (which knocks it out), and also causes the effect "If your opponent has a Stadium card in play, discard it".
It is clear that the opponent's Pokemon is knocked out before the stadium card is discarded (to the Lost Zone).
What is unclear is whether the effect of the stadium card still applies to any prize card(s) the attacking player draws for the knockout, or if the effect of the attack removes the stadium card from play before the prize card(s) are drawn.