If the target creature is an illegal target as Y'shtola Rhul's ability tries to resolve, it won't resolve and none of its effects will happen
In the meantime, though, Ill be trying to teach myself some Delver of Secrets
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Against a deck like Infect, though, even Vapor Snag can manage to do this work without the loss of a card
That's always welcome
Many owners look down on this, although it happens all of the time