I know this doesn't quite fit the pattern, but could there be Denarian or members of the Circle in Storm Front?
I /absolutely/ believe "the Circle" or "the Black Council" (or some agent(s) of theirs) was behind things in Storm Front.
As you say, the Heart-Ripper was likely from the Red Court, who Harry pegged as a Black-Council catspaw.
But it could also have been Papa Raith, who seems to have had a copy of the spell (was planning to use it on Thomas and Harry, to break Mama-Maggie's death-curse). But we suspect from Raith's magic-immunity ward, and know from the appearance of an Outsider at the big curse at the end, that Raith had Outsider backing... which we suspect points to Circle/Blouncil involvement.
I wonder if the Reds got it from Raith, or vice versa... or if each were given it by a 3rd party (e.g. the Circle).
Three-Eye was another OP tool given to Sells; the effect is too potent and too precise to be the formula of a rando-warlock newly come to power (as per his wife's testimony). A good potion like that (or at least, the
formula for it) is really
hard to create!
It's possible Sells just stumbled into some Grimoire of the Toad, and the Lightning-Fuel Spell, but the pattern is that Sells was being fed a bunch of OP tools.
Note the repeating pattern, BTW, of "minor/moderate badguy gets a serious powerup from being given OP tools." Burnt-out FBI agents get hexenwulf belts; Bianca gets Mavra-tutored, Kravos gets a power-up'ed (twice!), etc...
I'm... not really seeing Denarians anywhere in this. Except maybe Thorned Namshiel, who seems to be the best "wizard" amongst the Denarians, AND a member of the Black Council... and thus may have been the agent who created the Heart Ripper and other magics, and gave them to designated agents of the Circle, or the Blouncil, or whatever we're calling it this week. Of course, we suspect several very-senior & very-powerful human wizards (e.g. Cowl) are also on the Blouncil, and could equally-well have done it without calling Nammie for any "pointers".
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Also, I'm pretty sure we have WoJ that the whole series, including Storm Front, is a contiguous arc; not just "Harry's Story" but all part of the Bad Guys' grand plot (I mean, besides the grand "Jim Butcher gets rich and famous" plot).