If either or both targets are illegal when Prey Upon tries to resolve, no creature will deal or be dealt damage
Being a somewhat casual player, I must agree
Continuous Spells A Continuous Spell Card stays on the field once it is activated

Ignite!" When Lumis and Umbra activate "Chosen One", Lumis announces "I'll risk my Duel on this card!", followed by "I activate the Spell, Chosen One!" When Lumis and Umbra activate "Curse of the Masked Beast", Umbra announces "I'm using this Spell Card!", followed by "The Ritual Spell, Curse of the Masked Beast!" When Lumis and Umbra activate "Mask of Brutality", Lumis announces "The Equip Spell, Mask of Brutality!" When Lumis and Umbra activate "Mask of Dispel" or its effect, Lumis announces "The Continuous Spell, Mask of Dispel!" When Lumis and Umbra activate "Mask of Restrict", Lumis announces "I activate the Continuous Trap, Mask of Restrict!", followed by "As long as it's on the field, we can't Tribute cards!" When Lumis and Umbra activate "Mask of the Accursed" or its effect, Lumis announces "The Equip Spell, Mask of the Accursed!" When Lumis and Umbra activate "Mask of Weakness", Lumis announces "I activate my Trap Card

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If this card is Special Summoned during your opponent's turn: You can Tribute this card, then banish 1 card from your hand or face-up field