The big underlying threat of the setting is the Idea Of Thorns; a memetic virus that's responsible for the gardens being abandoned in the first place. You'll only encounter it very deep in the gardens, and before you do it's likely you'll encounter stuff that foreshadows it in minor ways. Potentially a LotFP-style campaign-warping disaster. Inspired by the Rapture in VotE, but taken in a different direction.)
The Idea Of Thorns is not a physical creature. It is, instead, somewhere between a disease, a hostile meme, and a spiritual presence. It is conscious. It wants to spread.
It does not need host minds to exist, but must find minds to infect if it wishes to affect the physical world. It will try to infect one mind, overtake it utterly, and then spread. It isolates, infects, subverts and controls.
When the Idea Of Thorns is first encountered, it will be as information. Writing scrawled on a wall, a little notebook with poetry in it, the sound of distant singing.
Anybody who refuses to look, covers their ears, etc, is not attacked by the Idea.
Those exposed to the Idea must save vs Magic. Any who fail have the first seeds of the Idea planted in their mind. At this stage, the only sign is that they see all plant life as having thorns. The thorns are real to them, and can cause damage.
Hearing somebody infected with the Idea talk about the Idea or their experience of it causes you to make a Save to avoid infection.
The Idea can compel an infected victim to take an action: roll a d20: if the result is equal to or higher than the victim’s Sense of Self (which equals charisma + constitution), the victim performs whatever action the Idea wishes.
The victim does not realise that the compulsion originated from outside their own mind.
The Idea will attempt to lower its victims’ sense of self by attacking the victim’s minds in the following situations:
à If a victim experiences an altered state of consciousness, such as drunkenness.
à If a victim sleeps.
à If a victim tries to work out an explanation for the Idea.
à If a victim tries to express the Idea or their experience of it to somebody not yet infected..
When one of these happens, all those nearby who are infected experience a sudden vision. They are faced with the Idea Of Thorns, incarnate as a towering being made of tangled vines, its form mimicking those whose minds are infected. It has all the moustache-twirling villainous bombast you can hope for. They must fight it. The fight lasts a single round and then ends. The next vision continues the fight where the last left off.
Treat the Idea Of Thorns as having the following stats:
à HD as the highest Intelligence out of those infected.
à 2 HP per HD.
à Armour Class is the highest Wisdom out of those infected.
à 3 Attacks with a bonus equal to the highest Charisma out of those infected.
à Saves as a fighter, level is the lowest Wisdom out of those infected.
à Immunity to mind-affecting effects. Immunity to poison. Double damage from fire and electricity.
The Idea does not attack hit-points, instead it attacks Sense of Self. Each successful hit halves the victim’s Sense of Self (round down).
As the victim’s sense of self falls, they will be prone to increasingly irrational behaviour.
If a victim has lost any sense of self, the Idea will make the following actions seem appropriate:
à Producing texts and artworks that express the Idea Of Thorns.
à Killing those that seek to oppose the Idea Of Thorns.
à Planting roses, briars, thistles and other thorny plants in places where they will flourish.
à Abandoning the trappings of civilisation and returning to a more feral state.
à Destroying buildings and replacing them with plants.
A victim who does one of these in a way that amuses, impresses or surprises the GM can, at the GM’s whim, earn an XP reward. The amount of XP granted is 50 multiplied by the total amount of Sense of Self they have lost.
If the victim’s sense of self reaches 0, they become an NPC, totally enslaved by the Idea.
If the Idea Of Thorns reaches the mortal world:
This is bad. It will spread exponentially quickly, rapidly taking hold among the common civilians who have comparatively little defence against it.
à After one day, several NPCs the players have spoken to are infected. They behave strangely, abandoning their normal lives. Hunting down and dealing with those infected to prevent the idea’s spread is reasonably practical. Minor surreal magic occurs; plants grown where they shouldn’t.
à After three days, a large amount of NPCs in the local settlement are infected. Graffiti expressing the idea springs up in public places. Arrests for crimes such as vandalism and arson spike massively. Hunting down those infected is still possible, if difficult. More surreal magic manifests, such as strange weather.
à After one week, the idea has spread to other settlements. Major powers in the world become aware of the problem. The settlement first infected is basically fully-infested by the Idea. Buildings are destroyed, plants begin growing over the ruins. The infected turn on one another. The only practical way to stop the infection is to burn it out with massive collateral damage. The dream-like effects on reality become pronounced. Plants move at night, animals behave weirdly.
à After three weeks, other local settlements are in an uproar. Major powers in the world send serious force to deal with the problem - armies, inquisitors and skilled mages. Pitched battles ensue. Brutal force or inventive tactics can still contain the infection, at great cost. The world becomes increasingly odd and dream-like magical effects manifest frequently.
à After a month, attempts to contain the Idea fail. Those infected run rampant. The world shifts and warps under the strange magic released. The infection cannot be contained.
à After three months, the world begins to resemble a slow, surreal zombie apocalypse. Plants grow everywhere, those infected who survive form gibbering packs, settlements are in ruins, the world follows dream-logic rather than the laws of science. Survivors cluster together in paranoid communites for support.
à After one year, survivors form xenophobic closed communities, cut off from the outside world. Outside these fortresses of sanity, the world is a plant-choked wasteland, as surreal as any extra-planar space.
à After three years, the communities of survivors begin reclaiming land from the Idea, slowly and carefully.
à After a decade, the Idea is pushed back and contained. The world becomes somewhat less surreal, although the lingering effects never truly leave. Society starts rebuilding, after the massive upheaval that was the idea. Sporadic outbreaks every few years threaten this stability.