If you were to cascade into a spell like Surprise Deployment during your main phase, what would happen? Does the spell get cast as normal or does it fizzle out since it's not during combat?
"Can only" is another way to say "can't do, unless". And "can't" beat "can". So you can't cast Surprise Deployment at all during your main phase, not even out of a Cascade effect.
It doesn't fizzle, you are just unable to cast it. It stays with the other cards you exiled until you reach it, and ends up going to the bottom of the library with them.
It doesn't fizzle, you are just unable to cast it. It stays with the other cards you exiled until you reach it, and ends up going to the bottom of the library with them.
It's not during combat, so the "Cast this spell only during combat." restriction forbids casting it.