So Nair, Bthrow, or Dthrow into sinking skull is a thing. It's not a true combo, but I can see it as a great way to punish air dodges. The spike hitbox is pri rediculous. It reminds me of his Dair from brawl. His body doesn't even have to come anywhere near you for him to land the spike.
As an added plus, Fly pops them out of the ground before landing the final hit, so you get the full KB. I can see this being paired with Rising Cyclone for extremely early kills.
For even more fun, possibly consider tossing in blast burn and punish mistakes with extreme prejudice
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A Few other things:
If Charizard has any downward momentum, sinking skull will automatically cause him to fast fall.
If Charizard is already fastfalling, sinking skull will cause charizard to revert back to regular fall speed during start up, then he will be put in fastfall again after the attack.
It's a small point, but it can possibly help throw opponents off when trying to land
He doesn't completely sacrifice a landing option by taking this over Rocksmash
The spike seems to launch at a slight angle. However, This could've been the result of CPU DI.