This card’s [On Play] effect reveals cards from the top of my deck. If only 1 card has [Undead] or [Dark Animal] in its traits, can I choose whether to add it to my hand or trash it?
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Official Ruling
No. For this card's effect you reveal cards, add 1 card with [Undead] or [Dark Animal] in its traits among them to your hand, then trash 1 card with [Undead] or [Dark Animal] in its traits among them.Therefore, if there is only 1 card with [Undead] or [Dark Animal] in its traits among the revealed cards, it's first added to the hand, so you can't trash it.
Source: BT9-071 Dracmon