If "Sacred Phoenix of Nephthys" is destroyed while it is not a Monster Card, such as when it is equipped to "Relinquished" or "Sword Hunter", its effect does not activate and it is not Special Summoned.
Which means, if she is the only monster on the field, you MUST use the effect on Tsukuyomi, and she would not return to hand since she is now face-down on the field and her effect of returning to hand cannot resolve while face-down.daivahataka said:2. Since it's text is "When this card is Normal Summoned or flipped face-up, select 1 face-up monster on the field and flip it into face-down Defense Position. ", and not "..., you may select..." it's mandatory.
daivahataka said:1. Ruling No. 7 under Phoenix in the card registry:If "Sacred Phoenix of Nephthys" is destroyed while it is not a Monster Card, such as when it is equipped to "Relinquished" or "Sword Hunter", its effect does not activate and it is not Special Summoned.
2. Since it's text is "When this card is Normal Summoned or flipped face-up, select 1 face-up monster on the field and flip it into face-down Defense Position. ", and not "..., you may select..." it's mandatory.