You choose the target for Soul Seizer's triggered ability when that ability triggers and goes on the stack. You choose whether or not to transform it when that ability resolves. If the creature is an illegal target by that time, the ability doesn't resolve and none of its effects happen. You can't have Soul Seizer transform.
2011-01-22
wotc
If Soul Seizer leaves the battlefield in response to its triggered ability, you won't be able to transform it and it won't return to the battlefield or become attached to the target.
2011-01-22
wotc
If Soul Seizer has any +1/+1 counters on it when it transforms, they remain on Ghastly Haunting. They won't affect the power or toughness of the enchanted creature, however.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Not Legal
Timeless
Not Legal
Gladiator
Not Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Not Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Not Legal
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