If I use this card's [Main] effect to place an opponent's Digimon under my opponent's other Digimon as its bottom digivolution card, will the Digimon to be placed in digivolution cards leave the battle area?
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Yes, it will leave the battle area. The digivolution cards of the Digimon to be placed in digivolution cards will be trashed at the same time.
Source: BT11-109 Astral Snatcher