When I play this card, do I have to trash 1 card in my hand?
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Official Ruling
No, the effect reads “you may,” so you can choose whether or not to trash the card. If you don't trash a card, you don’t get to return a Digimon card with [Seven Great Demon Lords] or [Three Musketeers] in its type from your trash to your hand.
Source: BT6-068 Impmon