The Phage and the Jelly
The Phage
Suspected by some to be a bioweapon engineered by fey warlords in an attempt to study the mortal soul (something famously lacked by fey), The Phage appears to be a fungal strain of great danger and great potential. Of particular interest to bastards and highly prized by the Brotherhood of the Open Eyes, a decentralized psychedelic cult, no one can quite recollect exactly when The Phage appeared on the prime plane. Since its introduction, however, it has spread across the realm, and while it is in its nature to connect, the vast mycelial webs have been broken in spots, leading to localized pockets of The Phage that attempt to spread their influence from their own isolated groves. The Phage itself can subsist as a mycelial network, dormant beneath the surface until such a time as conditions allow it to expand onto the surface. Once those conditions exist, mushrooms begin to fruit, and the spores released by the shrooms are capable of infecting living beings.
Phage-infected As Undead
The spores of The Phage are incredibly virulent and will implant in the lungs of those unfortunate enough to breathe them in. Onset is swift, and the fungus will quickly grow to the brain of the infected, and from there, infect the host's nervous system and take control of the body. Once the symptoms are visible, it is too late for the infected. The infected body will then seek to return to the "heart" of the mycelial network, and on reaching it, will allow itself to be digested by the network. It will generally shamble along with no goal other than to return, but if something attempts to stop it, it will defend itself and attempt to spread the infection by breathing a cloud of spores onto its attacker.
A phage-infected creature or person functions effectively as a zombie in other tabletop adventure games. Once they are infected, they are beyond help, and thus fill the niche of "thing we can kill without feeling bad about it".
Fungal Mothers
The Phage will infect any living creature, be it a worm, tree (entire forests have been known to succumb to infection), deer - it will consume anything with living essence with which it can sustain itself. However, something particularly sinister occurs when a sapient being is consumed by The Phage. The heart of the network will begin to flourish as a sort of fungal grove, with a tree-like structure dominating the area. This Fungal Mother begins attracting the lion's share of absorbed life essence from the network, and the spread of the network becomes more directed, seeking out concentrated sapience - civilization.
The reason Wizards and Shroom Priests are so interested in The Phage are these fungal mothers. As they grow they begin sprouting apple-sized fungal fruits, which are rich in life essence, and serve as "reservoirs" of energy for when feeding is infrequent. These fruits, when consumed, are highly psychoactive, and allow access to the life essence, memories, skills, and potentially souls of the poor fucks previously absorbed, for as long as the effects last (eating an entire fruit causes a trip of about a day), but maintaining focus long enough to put it to use is another challenge altogether as the psychedelia takes hold.
It should go without saying that harvesting these fruits is incredibly dangerous, as the grove seeks to defend the mother with phage-infected that have yet to be absorbed and the dispersal of unceasing spore clouds.
In particularly unfortunate situations, two mycelial networks may find each other, each with their own Mother. In these cases, the networks gauge their relative strength, and the stronger Mother becomes a Fungal Queen, allowing its vassal enough energy to sustain itself and spread, but redirecting the majority of the flow to its own grove. Given enough time, the entire region between the two mothers will warp into a hellish fungal forest, and as more Mothers are added, the horror continues to expand. The fruits of a Queen are fittingly more powerful, but their groves are correspondingly more dangerous.
The Fruit and the Jelly
Eating a fungal fruit raw has a 1/3 chance of driving a character irrevocably insane as they are assailed with the thoughts and memories (including the horrific memories of being infected and digested) of the unfortunate departed. It further has another 1/3 chance of lifting the veil from the character's eyes, allowing them to cast magic using Magic Words instead of drawing power from magical components; this effectively makes the character a Wizard, and unsuitable for play as outlined in my previous post (although at your table, do as you wish). If a character is lucky enough to not succumb to one of these two horrible fates, they have a day to plumb the memories of the dearly departed. Off the top of my head, I see this as usable in the same way as Nick's bookshelves, with the GM deciding which profession are represented in the fruit, and allowing characters under the influence to answer a number of questions pertaining to that subject as necessary.
The fruits can also be dried and preserved for later use. The same risks apply.
However, some deranged mycologists and horticulturists have learned that leaving Mothers' fruits near beehives will inexplicably cause the bees to abandon all previous work to harvest the jelly from the fruit, and bring it back to the hive . The jelly honey produced is filtered through the bees' primitive minds, removing the drawbacks but also diluting the power. This jelly, when consumed, induces a mild but still psychedelic trip during which the character can now cast spells from material components (this is a change I'm making as I'm trying to run a classless and levelless game, and needed a way to keep every Tom, Dick, and Harry from being able to cast), and can also see through the veil that normally hides the Fey realm.
Consider this an entry into the October Blog Carnival.