Actually, considering he did precisely that (he held off more or less the entire Council as a whole, mostly by himself), I tend to think he DID perform a Darkhallow (possibly one on a smaller scale than that attempted in Dead Beat)...and then wrote about it. It's never stated anywhere that he didn't, and it would explain his truly absurd level of power.
The darkhallow is the name for the ritual that involves summoning the erlking and using halloween.
THats the super secret pie recipie he came up with.
But its the same process he came up with and used in smaller scale: Eating ghosts and spirits for power
He'd been eating spirits, rampaging around europe during a world war, generally just doing his thing, for awhile. Thats gotta stack one with some mojo. Not to mention the whole zombie minions and the Mini-Archive he had plus whatever other items of power a centuries old dark lord of necromancy and wickedness that even scares MAB could accrue over a lifetime. Makes him truly absurd with the power, no darkhallow specifically needed.
All the darkhallow is are very efficient instructions to maximize your ghost munching potential. You don't NEED to call the erlking up, weaken the boundaries, and spin things into a vortex in a populated area to eat a ghost for power. Thats just a more effective way to do it.
Think of the darkhallow as the beer bong of ascension rituals involving necromantic power. It just makes it easier to get it all down, FAST
