Both the Darkhallow and McCoy's more spectacular feats of mass murder fall comfortably into Archsphere territory, I believe
The Darkhallow in particular was the same sort of thing that Archmage Voormas tried to pull in oWoD's Time of Troubles mega-event by eating a whole passel of souls to become a god.
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Why? Quiet uses of extreme skill arent exactly obvious, especially if employed intelligently.
If she was using Life 6 to turn people into Captain America-analogues and sending them off on missions in Europe and Africa and Asia, we wouldnt know it.
Hell, Peabody has been Senior Council secretary for more than twenty years. He was literally there in his official capacity when Dresden was tried for the death of DuMorne. Man has apparently had the access and ability to smuggle an Outsider into the middle of White Council HQ for years, and has done nothing with it besides keeping it in reserve.
There's no point displaying a capability if you dont think the juice is worth the squeeze.
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Summoning is usually ritual magic.
Its not all that hard for any significant faction of immortals to spend a couple years scouting and raising their own human warlocks for mortal-only magicks. You dont need Council-tier wizards for ritual magic.
The porn star strega that Lord Raith used to summon He Who Walks Behind in Blood Rites were all mortal.
According to Captain-Commander Luccio in Dead Beat?
Only mortal magic-users can summon Outsiders. Explicit statement.
Could be thralled sorcerers; unlikely, but possible. Marcus was cultivating Silk for a reason, after all, and in South America proper he wouldnt have had to be nice about it.
Could be Infested/half-Red sorcerers. Could be rogue wizards.
Plenty of options.
Like I said to
@firefrog600 , He Who Walks Behind got back into Earth because he was summoned by Madge Shelly and the rest of her coven of black magic witches in Blood Rites, none of whom were particularly impressive in strength.
Its an authority and knowledge thing, apparently. Not a power thing.
Worst case, you burn a human sacrifice or two. And evil factions have never had an issue killing people to fuel black magic.