While all of the occult classes have some connection to strange otherworldly forces, the medium is the only one crazy enough to allow all sorts of spirits to enter his body and mind in order to share their strength, allowing the spirits to gain more and more influence over him in exchange for great power. The playtest medium's spirits were based on the cards of the Harrow deck. Since that meant 54 spirits, each with unique abilities, that proved far too lengthy to fit in Occult Adventures without cutting, so I've saved that work, and we're hoping to release it in some venue.
The medium in the final book, however, has a new theme and direction, bolstered by countless ideas from playtesters that didn't fit with the Harrowed medium but work great in the new paradigm. In the new approach, the medium's spirits are born from the Astral Plane, where the Astral stuff clings to tiny fragments of the souls of people that pass through it and blends them with the tales and thoughts of others about those people. Spirits of power thus arrive from one of six different sorts of legends, which share their names with mythic paths (archmage, champion, guardian, hierophant, marshal, and trickster). Within those categories, a medium might find countless spirits with slightly different abilities. For example, one trickster spirit might be able to help with sneaking and another with grifting. A rondelero duelist champion might be a master of the falcate, and a samurai champion might know how to use a katana. Because these six legends correspond strongly to party roles, this change allows the new medium an unparalleled ability to fill in whatever role you need that day. Going to a masquerade ball? Bring a trickster master of disguise. Party missing a cleric today? Bring a hierophant and literally add the divine spells you need to your spell list that day. Same with archmage for arcane spells. And while the medium is still sometimes a 4-level spellcaster, both the archmage and hierophant progress him up to a 6-level spellcaster!
The Harrowed medium built a special spell list solely from his spirits, but the new medium has his own spell lists, with some intriguing early-access options fitting of a 4-level spellcaster. Furthermore, he is the only 4-level spellcaster currently in the game that receives cantrips at all (and he gets them at 1st-level). This adds a lot of versatility to his options if he chooses a more martial sort of spirit, and it fits with the fact that he can temporarily gain 1st-level spells too, if he picks the archmage or the hierophant.
The medium could always use seances to bring spirits into himself and grant benefits to allies, and the new medium can still do that, but he also gained some special new types of seance that let him do things like trance at a location to learn of its tragic past or even channel a haunt into his body so his party can try to reason with it. The medium can now accept strange taboos from his spirit to gain extra benefits, causing him to act in unusual ways that befit the spirit's temperament. Lastly, the medium's astral ties allow him to perform other incredible feats, like at-will astral projection at 14th level!
But wait, you say, what about the archetypes? If the medium changed that much, how do the archetypes work? To make sure that the archetypes were an absolutely perfect fit for the medium, after the changes, I wrote them from scratch myself. And boy, some of them do some crazy and fun things! The kami medium (whose awesome art graces this blog) channels the spirits of kami instead of legends, losing all the Astral abilities for the ability to use strange ofuda and shikigami, and even gain a ward like a kami does. The reanimated medium is a medium in reverse: after dying, he borrows from his own potential future legends to come back and possess his own body, keeping himself alive. This unique state grants him a variety of benefits and unusual trade-offs. The relic channeler possesses particular relics of a select group of spirits, meaning she contacts the same ones each time. She gains object-based powers and receives extraordinary benefits in exchange for the lack of flexibility. The spirit dancer shakes the whole system of spirits on its head; instead of choosing a spirit each day and gaining its powers for the whole day, he selects a suite of spirits and spends rounds from a daily pool, allowing him to slip in and out of his suite of spirits with ease, in a strange and exhilarating dance. The storyteller is more obsessed with the stories and legends than she even is with the spirits themselves; they are a means to the end of discovering more about the tales of the past. She gains lesser benefits from her spirits in exchange for enhanced knowledge, story-based abilities, and some of the most powerful applications of bardic performance (like inspire courage). Also, she can send you into a story of her own making forever at 20th level, which is pretty nasty!
So however you enjoy letting strange spirits take your body for a test drive, Occult Adventures and the medium class have something right for you!