Necrotic Plague's last ability triggers when the enchanted creature is put into a graveyard for any reason, not just when it's sacrificed due to the ability it grants to that creature. It triggers if it and the creature it's enchanting are both put into the graveyard at the same time, or if the creature it's enchanting is put into the graveyard but Necrotic Plague isn't. (In the second case, Necrotic Plague is then put into the graveyard as a state-based action.)