The Archfiend ATK increase effect activates during both player’s Standby Phases. The increase only lasts until the End Phase of that turn.
If the “Infernalqueen Archfiend” is removed from the field or turned face-down after the effect has resolved, the Archfiend that had its ATK boosted will retain that attack increase until the End Phase of the turn.
If an Archfiend that has received the ATK increase from the “Infernalqueen Archfiend” is turned face-down or removed from the field, the ATK increase is lost.
You do not have to select the same face-up Archfiend monster during every Standby Phase.
“Infernalqueen Archfiend” may target a different Archfiend monster each Standby Phase.