Registered: Aug 2014
Location: The balance of the Force
He's not fvcking Sephiroth. Sadow and Keto used the Corsair to fvck with the core of stars and cause them to go supernova. It had nothing to do with alchemy, pretty much any decently powerful dark-sider could use it.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
The Supernova ability is unique to the star crystals. And the author who wrote that story said that "no jedi or sith is that powerful" when asked if a force user could cause a supernova. My interpretation is that he thought of the corsair as a "holy grail" kind of thing, some impossibly powerful object that allows the characters, for the sake of the plot, to do things which have never been and will never again be done.
As far as in-universe explainations are concerned, I'd argue that since sith alchemy has been shown to be somewhat of a science, with learnable rules and complicated recipies, that Sidious would be able to do it by creating a ritual which had never been created before.
So yeah, if you want to believe, I guess you can, but story-wise, the supernova ability is very "non expanded universe," in the sense that it exists solely for the purpose of one story and doesn't extend outside of that to other ones. For good reason, too.
Oh, and yes, of course Sidious knows alchemy. He's done it more in the comics and books but one of the later episodes of TCW did show him performing a ritual to create a "dark illusion" through the force bond between Yoda and Dooku.