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This based on the Obliviax, a monster that showed up in the 1e MM2. It’s completely changed for this edition, though.
Oblivion moss, sometimes also known as memory moss, is a sapient predatory plant that lives in the depths of old forests. It looks like a carpet of moss, but it can perceive its surroundings and move around slowly in search of prey. The creature feeds on memories, which it strips from the brains of its victims. It derives sustenance from those stolen thoughts, and can also use them to spawn humanoid simulacra that help it defend itself and find more food.
The memory stripping process is not actually fatal or permanent, but it’s still very traumatic. Stolen memories return at different rates. An adventurer’s knowledge of combat spells and maneuvers might come back after mere seconds, still in the middle of the fight. Important personal memories (“who am I?”) come back after a day or two, but smaller details (“what does my mother look like?”) can take weeks to be restored. Of course, the moss and its constructs are quite capable of fighting to kill, and might choose to kill a victim anyway after stealing their memories.
Dryads and Treants cultivate patches of oblivion moss close to their own territories, as a security measure. Invaders who stumble upon them have their memories ripped away and end up turned around, leaving the forest without knowing what they were doing or where they went while inside. Even if the knowledge comes back later, very few people are keen on repeating the experience. As the book says, in places where oblivion moss grows, lost ruins tend to stay lost.
You can turn the memory stealing around, too. The remains of a slain patch of oblivion moss retain memory fragments from its past victims, and if someone eats those bits they can acquire those memories. While the fragmentary memories of a bandit might not give you their entire guard rotation or the map of their stronghold, they might give you the combination to their main vault or to a secret door. That can be useful, though eating a sapient being presents obvious ethical problems.
We get a bunch of different stat blocks here, one for the moss itself and five for the humanoid minions it can build from stolen memories. The minions can be generated mid-fight by the moss, or they might be placed as normal monsters in an encounter with or without their master.
Oblivion Moss Mindmaster
This is the moss itself, a Large Fey Magical Beast (plant) and a Level 12 Elite Controller with 250 HP. It perceives the world with Blindsight 20, and moves around at a glacial ground and climb speeds of 2. It does have Forest Walk, though, so it’s not further slowed down by difficult terrain in its natural habitat.
The moss projects a Mind Blindness Aura (1) that makes it invisible to enemies that start their turns inside until the end of the enemy’s next turn. It can defend itself in melee with a Corriding Touch that does immediate and ongoing acid damage, but it really wants to use its psychic powers instead.
It has two basic ranged attacks: Scramble the Mind and Scour the Mind. Both to the same amount of psychic damage. Additionally, Scramble slides the target 5 squares and Scour makes the moss or one of its allies invisible to the target for a turn.
Its ultimate attack is Absorb Memories, a ranged attack vs. Will that does no damage. A hit prevents the target from using encounter or daily attack powers (save ends). Hit or miss, the moss then creates a mossling duplicate of the target, which uses the most closely matching mossling stat block (see below). This recharges when there are no created mosslings in the field.
The mindmaster can spend a minor action to Direct a mossling, allowing it to move its speed as a free action. And it can also Absorb Damage as an interrupt, taking into itself the damage from an attack that would hit a mossling.
So a moss mindmaster would only be able to keep a single mossling around at a time. If you want there to be more of them, add some as part of the initial encounter makeup. Those don’t count as “created”, so the moss doesn’t have to wait to use Absorb Memories.
Mosslings are Medium Fey Humanoids, and all of them are Level 12 Minions. They’re meant to be copies of people whose memories were absorbed by the oblivion moss - the creature is smart enough to seek out competent combatants for this purpose, since they make for more powerful mosslings.
Their signature passive trait is Mindmaster’s Thrall - if the mossling in question was created by an oblivion moss mindmaster, it acts immediately after its creator’s turn. They all have Forest Walk too, and a basic tendril attack that does minion-tier damage and has a rider compatible with their role. But the real star of the show here is the Simulacrum Attack, which allows them to copy an at-will power from a PC!
If the mossling was created by Absorb Memories during this fight, it will be a power from the PC that attack targeted. If you’re adding the mossling as part of the encounter, pick a PC whose class is similar to the mossling’s role and give them one of that PC’s powers. The attack targets AC and its bonus is based on the mossling’s level as normal, but the damage and all other effects are identical to the PC’s, as if it was the PC who hit. Some of them copy melee attacks, others copy ranged attacks, but the match-up is not a problem because you always use a mossling archetype compatible with the power you want to replicate, not the other way around.
The book recommends having the players themselves roll and resolve the mossling’s attacks, to reinforce the feeling that they’re fighting themselves. That’s clever!
Here are the different varieties:
Mossling Guardian: Soldier, speed 6. Has an aura (1) that slows enemies. Tendril attack allows it to shift 1 square and pull an enemy into the vacated space. Copies a melee power. Use to copy PC defenders.
Mossling Creeper: Skirmisher, speed 8. Deals extra damage with combat advantage. Tendril strike allows it to shift 2 squares. Shifts 2 squares when missed by a melee attack. Copies a melee power. Use to copy rogues and other melee strikers.
Mossling Hurler: Artillery, speed 6. Explodes on death, dealing a bit of psychic damage and inflicting Daze (save ends) on a Close Burst 2. No riders on tendril attack. Steals a ranged power. Use to copy ranged strikers.
Mossling Grower: Controller (Leader). Has an aura (1) that grants +2 defense to plant allies. Tendril attack slows for a turn. Gives 20 temporary hit points to every nonminion plant ally within 3 squares on death. Copies a melee power. Use to copy warlords and other leaders.
Mossling Vinecaller: Controller. Has an aura (1) that makes the covered spaces difficult terrain. Makes a Close Burst 1 in to difficult terrain for the rest of the encounter on death. Tendril strike slides target 2 squares. Copies a ranged power. Use to copy wizards and other controllers.
I think the mossling mechanic is really clever, particularly the bit where players roll and resolve their duplicate’s attacks. It helps ease the GM’s cognitive load a bit since mosslings are kinda finnicky even before you factor the copied attack in.
As I mentioned above, a moss mindmaster can only keep one mossling around at a time. If you want to have more of them in play, the easiest way is to add more of them as standard minions to the encounter. In this case I guess they would have their own Initiative rolls, and would be worth XP as normal, but the mindmaster could still target them with Direct Mossling and Absorb Damage.
Another option might be to turn the Mindmaster into a solo! Aside from the usual adjustments, you would want to make Absorb Memories at-will and remove the limit on the number of created mosslings in play. Maybe also turn Absorb Damage into a Free or No Action to make it usable as many times per turn as the moss wants.