

If “Skill Drain” is active on the field, the effect of “Sacred Phoenix of Nephthys” that Special Summons it from the Graveyard is still activated, because it is activated in the Graveyard.(For example, if an effect like “Star Boy” is active, and “Sacred Phoenix of Nephthys” is Summoned, and “Chain Destruction” is activated to destroy other copies of “Sacred Phoenix of Nephthys” in the hand and Deck, those copies will be Special Summoned during your next Standby Phase. If “Sacred Phoenix of Nephthys” is destroyed while in your hand or Deck by a card effect, you still activate its effect during your next Standby Phase to Special Summon it from the Graveyard.

If “Sacred Phoenix of Nephthys” is Special Summoned with “Call of the Haunted” or “Premature Burial”, and that card is destroyed and destroys “Sacred Phoenix of Nephthys” with its effect, you Special Summon “Sacred Phoenix of Nephthys” during your next Standby Phase because it was destroyed by a card effect.Because it is not destroyed, its effect to Special Summon itself does not activate again.

Because this effect activates in the Graveyard, and is being negated while "Sacred Phoenix of Nepthys" is in the Graveyard, "Sacred Phoenix of Nepthys" is never Special Summoned (so the card is never moved from the Graveyard) and is never destroyed.
