The player that controls Necrotic Plague at the time the enchanted creature is put into a graveyard is the player who controls the last ability (even though that creature's controller is the one who chooses its target). The first player returns Necrotic Plague to the battlefield under their control, regardless of whose graveyard it was put into. Note that the ability is mandatory; a target must be chosen if able, and Necrotic Plague must be returned to the battlefield if able.