Campaign Model: Growing strong
This campaign model tells the tale of a single powerful work of magic going awry. As such it's suitable for fantasy games. One could also replace the magic potentially with toxic or biological mutation or even a freak of evolution. The tone can easily be adjusted from light and adventurous to bitch black tragedy or horror.  As it stands, the adventure is designed with elements of mystery, horror, and fantasy. It's a slow build to a massive confrontation.
It all started with a simple tree. A beloved tree, with deep strong roots covered in moss and fungus. A tree that stood for decades, the favoured tree of a humble nature lover. And then there was a moment, a moment of beauty and a single instance of magic. Perhaps it was a blessing from above, perhaps the humble nature lover had earned the power through training or sacrifice or the admiration of a fairy. But regardless of how, they were blessed with the power to bring a plant to life, to sentience and sapience with a touch. But when the moment came to lay hand upon the tree, there was a mistake. Instead the magic flowed into the fungus, deep into the earth, spreading across a massive network of fungi so expansive that it covers much of the world itself. (And of course if it suits your game you can replace this origin with a network of fungal roots reaching a spilled pool of toxic waste.)
Now a single massive organism lives, and breathes and feeds and grows beneath the surface of the world. It hears words whispered in the royal gardens, it feels the indiscretions hidden in the cornfield, it drinks the blood of the battlefields and it comprehends all of it, constantly feeding an intellect as vast as the organism itself. It knows the knowledge of the open world, and through this it learns.
As it waits and listens, drinking in the knowledge of the world, it is discovered by a small coven who form a cult around it. They meet in tunnels webbed with roots and fungi and perform their strange rites in honour of the mushroom god they have discovered. Depending on the mood you want to go with these could be entirely mundane beings believing themselves mystical connected to this world sized life form, or they could have true magical power. Likewise, the tone can decide what exactly the cult is doing. A darker game could find the cultists abducting humans and using them as fertilizer to feed the their god, whereas a lighter game may have them pressuring others to join, stealing garden supplies and mulch, or consuming dangerous psychoactive in an effort to commune with the plant. But regardless of their motives and abilities, they do manage to catch the entities attention, and through their supplication it comes to believe that is indeed a god.
When the players inevitably foil the cult, this being takes notice of them, deciding to enact the godly vengeance that is promised by its cultists. It's first attempt should be minor but mystifying. It could be a great sinkhole opening up beneath one of the players, exposing a deep root filled tunnel, or it could be several small savage mushroom creatures attacking from a small hole in the earth. Even something subtler still, like the player's garden dying and rotting and filling with fungus in short order., and then finding the ground beneath the garden has opened up into small tunnels. Enough to tell the players that something is definitely going on, and to lead them to a network of tunnels similar to where the cult was, but smaller.
From here the players may want to wanted into the tunnels alone or they might group up to explore. Depending on how things worked out with the cultists they may seek out surviving members or victims. The important thing here is to put on more pressure. if they visit cultists they get waylaid on the way, by living fungi, felled trees, or more stronger mushroom minions. You may also choose to have the cultist or their victims also attacked, their fate also left to help enforce the tone of the game. If the players advance into the tunnels confront them with choking roots, or toxic spores, or animate mushrooms, nothing too strong for them, but enough to tell them they're on the right track. Another major boon of this model is that if the players try to leave or flee, the organism is everywhere.
If the players back off, give them a brief reprieve, let them catch their breath, plan their next move, and then as they implement that plan, put the pressure back on, no stronger than last time, but just a reminder that they can't escape this. If they press on into the tunnels heading deeper, then it's time to get psychological. The being communicates with them, and it wants the players to surrender to it. This can be anything from demanding they become it's champions, to submitting to fungal infection, to earnestly asking them to lay down and die amidst its roots. The important thing is that these are not the idle growling demands of a monster. They are the sweet cajoling words of a god, of a being so mighty that it already knows the party will die and fail, so they should save themselves the trouble. Sine few parties ever back down when asked to they'll likely press on. At this point wet stomp the gas and throw a big threat at them. Good options are a vast cavern with a massive fungal creature, a hidden clearing on the surface with fugally infected animals, optimally in varying states of decay, or just massive tentacle like roots. When the party wins the battle, they'll quickly realize the war is just beginning. This is a good chance for ominous messages from around the world. If players have friends across the world, have it drop one of their names and what they're doing. If there's a war the players have investment in, have the creature give ominous news from the front, or just tell them what their family back home is up to. This is a power play, this is the god revealing at least in part how expansive and powerful it is.
At this point the players should be allowed a little reprieve, a chance to draw back and consider their options. They can consider a lot of approaches here, Magical, Chemical, Alchemical, Psychological, the important thing is that as long as they settle on a somewhat reasonable goal, have it seem like it should work. If it seems like they really need a bone, you could even have them discover there's a powerful heart at the center of the mass, grown around where the spell was laid. Regardless of the approach as a general rule it should take 1 adventure to prepare and a second to implement. If they want to blow the heck out of it with explosives, that's 1 adventure to get the goods, and 1 to place and detonate. If the approach is magical it should be 1 adventure to find the info and another to bring it to bare. You get the idea.
It's also important to remember, the players by this point should be aware the creature can see and hear them in an outdoor environment. So if they decide anyways to prepare their plan outside near all the plants and moss and fungi, then take note. Have the plant react accordingly, it's not a stupid entity that will allow it to be destroyed. If it knows where the players are going and what they plan to do ahead of time, it should take steps to prevent it. whether that means taking over the chemical plant, infecting the sage who knows it's weakness, or anything else. Depending on campaign length, that could mean just making an encounter a bit harder, throwing in a few enemies or an extra obsticle, or it could mean completely foiling that option and forcing them to go on another adventure. That last option should also only be used if the players actually have more than one idea about how to stop the entity.
Finally all the preparations are set, the players have what they need and have finally begun moving to place it or enact it. At this point the entity offers a truce, a partnership, after all it is a being with more knowledge and power than any other , surely it could turn the tide of any conflict. Of course this being would never actually be the tool of mere mortals, but if it can buy enough time it can heighten it's defenses and defeat the players. Hopefully the players will see through this ruse, and together defeat the massive being, ending it's existence and saving themselves and perhaps the world.
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