"How many Rivers?"
Kerberoi guard the Old Laws,
& you must flee them.
Well, kids, they've done it again. These "seasonal" book series are just great; you can sell five books over the course of the year, & even if I don't like it (Changeling: the Lost or don't need the sourcebooks (Hunter: the Vigil) you still at least get one book out of me. Geist is the other breed: the kind that makes you anxious for the sourcebooks (Promethean: the Created)-- in this case specifically, whatever they've got coming out on the Underworld.
So what is a "geist"? Well first off; you are technically playing a "Sin-eater" though a Geist is the other half the equation...someone on the White Wolf forum summed it up as "me & my pet ghost," which really does sum it up. You are possessed by a "super ghost." Well, not possessed; you are merged with one. What the heck is a "super ghost," a "geist"? A ghost that makes the leap from specific to...archetypal. The geist isn't Sally, who died on the roadside of Route 306; nope, the geist is the Lonely Hitchhiker, the embodiment of the recurring wave-form, the conceptual distillation.
The Sin-eater is someone "touched" their whole life; maybe they had psychic dreams, or could see who would die soon, or hear voices outside of old buildings; whatever. Then-- they die. Maybe they have a near death experience, maybe they flatline on the operating table, maybe they snap their leg rock climbing & starve to death in the middle of nowhere. Then in the waiting room between life & death, a geist offers you the Bargin; die, or merge & live. The manner of death is the Threshold, the "clan" or "tribe" of the various Sin-eaters. If you die by violence you fall into one, or random chance, or deprivation, so on, so on. They don't have millenia old traditions; they have slap-dash pop culture, often each "krewe" specializing very uniquely. Sure, Mexican Day of the Dead stuff is very popular, but one krewe might temper is with Goetic sigils & Voudon ververs; another might slap some Lovecraft together with Tibetan bodhisattvas.
The powers? What the fuck. I think they might be...brilliant? The mechanic is such a departure, I'm still sort of trying to digest it. There is a two-part system, a matrix: your Manifestation is the real power, based on sort of classic ghostly themes; the Key is what unlocks the Manifestation. See, hard to explain. Okay, piece by piece. Manifestations cost New Dots x 6; Keys cost a flat 10 experience. The way it is lain out is Manifestation, & then dots spread down for each Key...ugh. Hard to grok, I know. Example: Manifestation "Caul" is the sort of physical alteration power; then for the Phantasm Key, you can turn yourself into real horrors; the Pyre-flame Key lets you burst into flame, so on & so forth. Clever, but tricky to slot into other games. Really promising though.
The Underworld. Yay, for any Underworld. So you get the Upper Underworld, the Autocthonous Depths. Then you get the Lower Mysteries, below that; & that is where the weirdness starts. There are Rivers; rivers of flame (Phlegathon), or bile, or scorpions, drawing on more than just Greek myth...they have benefits & draw backs when drank, hearkening back to the goblin fruit of Changeling. How many Rivers you need to cross to get to a place defines how far, how weird it gets. Your "Psyche," your Potency attribute, will allow you to cross them, one per dot; but once you can't go any further, you need a Boatman. What is going on in the Underworld? The Old Laws. Displayed in each Domain, specific to each-- "do not speak to the ghosts," "you must bring a music instrument with you," "you have to shed blood to leave." If you don't, the monstrous god-things of the Underworld, the Kerberoi, come after you.
So, what kind of a debt does it owe to Wraith: the Oblivion? Not much. There are hints of flavor in the Underworld; some of the Manifestations are very clearly callbacks to the Wraith's powers. What really gets me is how much of this seems almost lifted from my game! Tom's character, Bedwin, "died" & came back similarly merged. Sam's character has a Vice where he regains points of "Essence" from. The rivers of the Underworld. Etc. Great minds, think alike.