When I activate this Tamer's [Main] effect and do not digivolve into [MagnaGarurumon], can I instead digivolve into some other level 6 Digimon, like [AncientGarurumon]?
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No. With this Tamer's [Main] effect, you cannot digivolve into any other Digimon than [MagnaGarurumon].
Source: BT7-087 Koji Minamoto