Hi, I'm working on a homebrew class designed to be like a parasite that lives in the other players (inspired by the biophage kickstarter), acting as a unique type of support (though will have an ability early on to possess low level creatures to expand their versatility and open more playstyles) kind of like Yuumi that cat from league of legends.

The class is supposed to be a very customisable one (i'm a sucker for eldritch invocations), including replacements for species and backgrounds (origins and purposes), as well as mutations - basically just eldritch invocations - acting as the class's main source of features and giving it a highly customisable playstyle.

I've spent a lot of time coming up with rules on exactly how it'll work, and want to present the level 1 features of the class and hope some of ya'll can let me know if it all makes sense or not. Sorry if it's a lot to read, but it's basically replacing some pretty fundamental rules for most characters so it needs a lot of explaining.

Thanks very much for any feedback ya'll can give, preferably constructive criticism and advice on what could be rephrased/ improved.

LEVEL 1: Nature of a Parasite

You are a Parasite, a creature that must share the body with another creature to survive.

Due to the dependent nature of the parasite, the player may want to communicate how they begin the campaign. This can be done through communicating with another player that the Parasite will begin the campaign within, having the parasite be in some kind of storage that keeps it alive until it finds a host or even having the parasite form for the first time at some point early in the first session. At the DM’s discretion, the parasite could begin within an NPC host, using the rules of the Dominant Infestation class feature. In any case, the parasite should quickly end up within the body of one of the other player characters.

Origin and Purpose. You are a different form of life to most other characters and so do not choose a species or background when going through character creation. Instead you choose from an Origin and a Purpose, replacing species and background respectively.

Your Origin is in place of a species and is what type of being you are, with a range of creature types and abilities. Your Origin determines your true form, the body you own when outside of a host.

Your Purpose is in place of a background and is instead what you aim to be, how you intend to interact with the world around you and your host. Your Purpose determines your ideal form, the body that you would desire to have should you have your own independent body - or the complete control of another’s.

Symbiosis. While infesting a host, you count as being in the same space as them. You cannot be affected by physical effects. When your host is attacked, you take the same damage they do, though use your own resistances and immunities. You can still be affected by mental effects as your mind is separate to your host’s. When targeting your host with a mental effect, a creature can choose between you and your host. In the case they are not aware of your presence within the host or that you have your own mind, they default to targeting your host.

Your health is not usually damaged by attacks, however instead is primarily used as a resource for many of your mutations. This does not mean you cannot take damage at all however, as mental effects can still deal damage to you, likely in the form of psychic damage. Given your lack of a physical form, you also cannot get healing from normal means, making your  All Origins also have a weakness, a damage type that can disrupt your bond with your host. When your host takes damage of said type, you also take the same amount of damage. If you fall to 0 hit points while within a host, you fall unconscious but do not make death saving throws unless reduced to 0 hit points by psychic damage or your weakness, or take either damage type. You still begin dying when outside of a host.

True Form. Your true form is what you truly are when outside of a body. It is determined by your origin, though without the Wandering Form mutation your true form is incapable of any real action. Your true form has its own speed and size, and uses all your Ability scores and proficiencies as well as creature type. You also keep your hit points which are now at higher risk since you have a physical form that can be attacked. Taking on your true form causes you to lose concentration on any spells and you cannot cast spells, use mutations nor attack while within your true form.

When not inhabiting a host for any reason, you become unconscious and begin to decay and cannot become conscious again until you find a new host. You take 1d8 necrotic damage per turn that cannot be reduced or prevented in any way. Should you reach 0 hit points, you cannot be stabilized 

LEVEL 1: Control

Sharing control. You share a body with your host, both sharing control over it. When inhabiting a willing host, either party can act using the body. Should there be disagreement over a course of action, both parties make a Control check. Both host and parasite roll a check with their charisma modifiers, with whoever rolls higher taking temporary control, either performing their action or simply preventing the other from doing so. Conditions that affect the mind, such as frightened or charmed or any kind of mind control, only affects one mind, which allows the other mind to fight against any actions one is being forced to do.

In combat, the parasite acts on their own initiative, separate from the host. The host and them share movement, meaning however much movement isn’t used by the host on their turn, the parasite can use on their turn instead, and vice versa. Just like when outside of combat, if the host or parasite disagrees with an action, bonus action or movement action the other is taking, they can use a reaction to attempt a control check to disrupt the action. Should they succeed in disrupting the action, it is not done and they lose their action, bonus action or movement action for the turn depending on what type of action was prevented.

Not your body. When the parasite acts with the host’s body, any dexterity, constitution and strength checks and saving throws and attacks are made by the host, using their ability scores and skill proficiencies, but not weapon proficiencies. This does not mean that these stats are useless to the parasite however, as they are still used by mutations and for initiative in dexterity’s case. Should the host not be proficient in shields, the parasite can provide said proficiency, allowing them to wield a shield as long as the parasite is within them and conscious, the parasite in control of blocking attacks with it.

The parasite does not gain access to any of the host’s special features, such as those provided by their class, spellcasting or other magical abilities. They are able to use any action that is innate to the creature’s physical biology, such as unarmed attacks or different movement types, as well as any items they possess. If the parasite is capable of casting spells, they can do so through the host’s body.

If the host is knocked unconscious, the parasite is still capable of controlling the body, making all dexterity, constitution and strength checks and saving throws and attacks using the host’s stats and skill proficiencies, though all at disadvantage. The host still makes death saving throws as normal, and upon death the parasite is expelled from the host’s body. The parasite can attempt to stabilise their host. Should the host die, the parasite is forced out of their body.

If a host no longer wants you inside of it, it can attempt to force you out. They must make a charisma check against your charisma saving throw, and should they succeed you are forced out of their body. You are also forced out if the host is transformed into a creature type that you cannot Infest as described in the Infestation class feature.

LEVEL 1: Infestation

In order to survive, you must be able to take on a host, but at present you are too weak to fight against someone’s will, meaning any host you take on must be willing, so you gain access to the Willing Infestation action. By default, you are only able to infest humanoids.

Willing Infestation - As an action, you can change host to any willing creature you are eligible to infest. touching your current host or your physical form should you find yourself outside a host.

LEVEL 1: Mutations

While within the host’s body, the parasite is capable of manipulating their physical form, as well as perform a range of strange abilities unique to a being such as them. These abilities are known as mutations, and come with a type and a user. There are two types - benign, passive mutations that are always in effect, and malignant, mutations that must be activated. The user describes who benefits from benign mutations and who can use malignant mutations - the host, the parasite or a shared mutation which either affects both or can be used by both.

You gain 2 mutations of your choice from the list of mutations that you meet the prerequisites for.

Prerequisites. If a mutation has a prerequisite, you must meet it to learn that mutation. For example, if a mutation requires you to be a level 5+ Parasite, you can select the mutation once you reach Parasite level 5.
Replacing and Gaining Mutations. Whenever you gain a Parasite level, you can replace one of your m mutations with another one for which you qualify. You can’t replace a mutation if it’s a prerequisite for another mutation that you have.
When you gain certain Parasite levels, you gain more mutations of your choice, as shown in the Mutations column of the Parasite Features table.

You can’t pick the same mutation more than once unless its description says otherwise.