Well, the following is my very rudimentary attempt at a first draft:
Three things to keep in mind: 1) This doesn't include the complexity to summon up the Erlking and initiate a Wild Hunt (itself likely in the neighborhood of 14+ complexity, and requiring the proper chant/tune); 2) Nor does it include the cost of establishing a city-wide threat; and finally 3) It requires GM approval to even gain The Word of Kemmler.
Okay... fourth thing: I treated the Darkhollow as having an effective Conviction of +15. Therefore 20 shifts to summon it, and 56 shifts (if I did the math right) to bind it.
Darkhollow Ascension Ritual Or: How To Become A Minor God (at least) If You Don't Die (By Getting Your Fool Head Cut Off) First
Type: Thaumaturgy, Summoning/Binding/Transformation (necromancy)
Complexity: 99+ shifts (Note: Someone with the Aspect Possesses The Word Of Kemmler is treated as being only 6 shifts short of prepared, those shifts can only be made up by tagging the special Aspects mentioned in the Notes section)
Duration: Until death (it could happen...)
Effect: The recipient of the Darkhollow gains all of the following:
-Master and/or Imbiber of the Darkhollow (or something to equal effect) replacing one of their permanent aspects, most likely their high concept...
-All of their skills are "bumped up" 10 levels, effectively ignoring the rules of the skill ladder by sliding the "base" of that ladder up ten levels.
-Mythic Recovery and Toughness with The Catch [+0] as theoretically unknowable (though if one were to completely consume your powers...) and Physical Immunity with The Catch (stacked) [+0] as presumably powers of other beings in your new supernatural weightclass.
- 30 Refresh Points to be immediately spent (along with any additional Refresh and Fate Points possessed by the character) on any combination of powers from the following list (including those developed on the spot and accepted by the GM): Psychic Abilities, Shapeshifting Abilities, Speed, Strength, or Spellcraft (though Refinements purchased this way cannot grant Focus Items or Enchanted Items)
Notes: The special Aspects which need to be in place before the ritual can be initiated are: Possesses The Word of Kemmler (needs to be tagged to make the ritual possible), Ongoing Wild Hunt (effectively a scene aspect), It Is Halloween, and a city-wide aspect of at least Threat level pertaining to Death and/or Fear which came about as a direct result of preparing for the ritual.
If, at any point, the caster should receive backlash from the ritual it is instead all expended as fallout, striking about a city block's worth of zones.
Finally, the astute reader will notice how this delightfully plot device ritual will turn the caster, should he survive it, into an NPC.