You are a non-physical life-form that lives in the mind. You are an idea that has gained consciousness and self-determination. Although your survival depends on the survival of the host carrying you in their mind, you may be able to gain immortality by lodging yourself into another mind.1)
Game Master Approval
Like most character choices, choosing this ranked ability requires approval from your Game Master. However, given the strange and powerful nature of this ranked ability, that approval must be explicit and enthusiastic - you must ask your Game Master some variation of the question: “May I play a character who has ranks in the Thoughtform ranked ability?”.
Ideology
You must be A Thought - something that your host believes to be true. For example: “Fenris is a giant wolf who will devour the world during Ragnarok”, or “the government is run by a secret cabal who hides the existence of alien influence”, or “people are generally good”, or “God exists”, or “God doesn't exist”, or “all fish swim”. Crafting this belief requires a conversation with, and approval from your Game Master. If this exact belief is shared by another creature beyond your original host, then you exist in both your original host's mind and the new creature's mind, simultaneously - this is how you ensure your survival. Your existence in multiple minds is not like the existence of multiple copies or clones, where those copies are separate or independent, you are simply a being that exists in more than one place at a time, and your 'host' is no longer a singular being but instead counts as all those beings who believe in you. You are not allowed to violate or act against this belief, if you do, you risk suffering injury and the death of being forgotten.
Limited Influence
Communication with your host is limited and interaction with the world is impossible. You can give your host a vague emotion or sensation to try and communicate or influence them. If you spend an inspiration point, you can communicate to them as a 'voice in the back of their head' for the next ten minutes.
Non-Physical Life-Form
Your existence is entirely dependent on the existence of your host or hosts. If the last host to believe in you dies, you die as well. You must either protect your host, or get another host to believe in you before your original host's death. Despite the fact that you are not physical, you can still suffer injury - thoughts can be changed, or misremembered, or even forgotten - if something like this happens to you, you risk death.
Shared Senses
Since you inhabit a mind, information you have access to is limited to what your host knows and senses.
Hosts
The maximum number of hosts who believe in you is limited to your ranks in the 'Thoughtform' ability, as follows2):
Each time you gain ranks in Thoughtform, you may also gain your choice of one of the following perks3)4):
Apparition
Prerequisite: Thoughtform rank 1.
As an action, you can create an illusory representation of yourself to communicate more easily with your host. This appearance must take the form of a physical creature and cannot change (other than wardrobe changes or those changes necessary for bodily or vocal communication). Only your host can sense your apparition, but you can use this ability to appear to your host at any time, even if they are asleep or unconscious. This ability does not allow you to generate other illusions - including writing, a heads-up-display, or obstructing your host's senses if they do not want them to be obstructed. Until you have this ability, communication with your host is limited to vague notions or emotions, or a “voice in the back of their head”.
Compulsion
Prerequisite: Thoughtform rank 1.
You can spend 1 inspiration point to compel your host5) to perform an action, even if they don't want to do it6). Your host must have an action available to do so. If you use this ability to compel your host to perform an action that they are incapable of, they automatically fail.
Pleasure Center
Prerequisite: Thoughtform rank 1.
You can create one inspiration point that your host can use. You can use this perk a number of times per day equal to your ranks in the 'Thoughtform' ability.
Hunch
Prerequisite: Thoughtform rank 1.
You can successfully intuit the motivation of a creature that your host can see, hear, or otherwise sense. You can use this perk a number of times per day equal to your ranks in the 'Thoughtform' ability.
Extra-Sensory Perception
Prerequisite: Thoughtform rank 1.
You can sense the world around you independently from your host, as if you had your own eyes, ears, and other sensory organs. What senses you have is limited to what senses your host has. The area that you can sense is limited to the position of your host - you cannot sense anything as if you were standing somewhere else.
Sleepwalker
Prerequisite: Thoughtform rank 1.
While your host is asleep, you can take full control of their body and senses. Your host's mind remains dormant and passive during the time when they would normally sleep, however: Any strenuous activity; attempt to wake them; or startling event nearby will end your control over their body and senses and return their own control. Anything experienced by the host during this time is usually forgotten, though sleepwalks, if repeated often enough will cause the host to feel fatigued, and start to remember those experiences - first as if they were a dream, and then the host will begin to suspect more and more that they are real and motivate the host to decide that something is wrong and fix it. A conscious agreement with your host can be enough to let them cooperate with your control.
Psychonaut
Prerequisite: Thoughtform rank 1.
You can, with some effort and time, navigate through your host's mind and learn more about the host than they know themselves, since you can explore the unconscious reaches of their mind better than they can. However, while doing so, you may encounter mental defenses, animated traumas, fears, and nightmares that will injure or destroy you.7) While navigating a host's subconscious your presence is undetectable to outsiders, even if they use special powers like telepathy.
Your ranks in the ability 'Psychonaut' are treated as if they were at least 2 less than your ranks in 'Thoughtform', to a minimum of 1 rank in 'Psychonaut'. This ability does not grant perks from the Psychonaut ability.8)
Projection
Prerequisites: Thoughtform rank 1; Thoughtform perk: Apparition.
You can cause your illusory self to be seen and heard by other creatures, not just your host. This illusion is limited to the same limitations as 'Apparition'.
Guardian Angel
Prerequisite: Thoughtform rank 1.
You can, once per day, protect your host from death. If something were to cause their death, you can invoke this ability to allow them to escape that consequence. If you have the 'Apparition' perk, your illusory self can also generate illusions that explain how your host escaped death. If you have the 'Extra-Sensory Perception' perk, you gain the ability to sense your environment as if you were standing someplace different from your host - you can choose to treat your senses as if they were centered up to 100 meters away, even if that location is out of line of sight from your host.