Vessel

Your magical ability is not your own. Whether through parasitism, symbiosis, or an otherworldly presence, your body is a vessel for some external force or entity that channels its power through you. Perhaps you engaged in an amicable arrangement with a bodiless consciousness, encountered a force that overwhelmed and invaded you, or serve as a vehicle for something unknown even to you. However you came to your current state, the powers you now wield allow you to exert magical power upon your surroundings, as long as the shell of your body maintains the constitution to do so. Whether those powers will be used for your ends or those of your Passenger is another matter entirely...

Durable Husk

Starting at 1st level, your exposure to the Passenger you carry within you has changed how you process your magical ability. 

Constitution is now your spellcasting ability for your sorcerer spells, since the power of your magic relies on your ability to maintain your physical composure against the forces pouring out of you. You use your Constitution whenever a spell or feature refers to your spellcasting ability. In addition, you use your Constitution modifier when setting the saving throw DC for a sorcerer spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your Constitution modifier

Spell attack modifier = your proficiency bonus + your Constitution modifier

What Dwells Within

The nature of your status as a vessel is dependent upon the nature of your Passenger and the relationship you have with it. When you take this Sorcerous Origin at Level 1, you choose the nature of your Passenger from the table below. Your choice will impact what benefits and effects you receive from subsequent Features acquired by this subclass. At the DM's discretion, you may choose a different option later in the campaign if your relationship with your Passenger changes. 

Parasite. You are an unwilling host to an invading entity, force, or power that has somewhat (or wholly) taken over your physical body. Your mind may remain fully intact, but your body is no longer completely your own to command. 

Symbiote. You have allowed an entity to coexist with you inside your physical body. While this can range from an amicable arrangement to a tense compromise, you share agency of your physical being with another consciousness, at least somewhat willingly. 

Anomaly. You are not sure of what it is that lurks within you. Your powers manifest, but your knowledge of how and why is almost completely absent. Perhaps a consciousness completely alien and unknown to you, or not a consciousness at all. You are host to an enigma. 

Mortal Conduit

Starting at 1st level, the latent power of the entity within you manifests as changes to your physical being. The effects are determined by your Passenger type.

Parasite. Your physical strength is drained in order to feed your Passenger’s power, leaving your skin dry and heavily cracked and peeling in some areas. You have a penalty equal to your proficiency bonus on all Strength and Dexterity checks and saving throws. In contrast, all of your spell attacks do additional damage equal to the maximum of 1 of the spell’s damage die plus your proficiency bonus.

Symbiote. Your Passenger lends you its awareness and mental prowess, causing your eyes to grow second pupils that glow slightly. Your proficiency bonus is doubled for Perception and Investigation checks, as well as Wisdom saving throws and Constitution saving throws to maintain concentration on a spell. 

Anomaly. The strange, otherworldly nature of your Passenger seems to affect the very elements of the Weave, infusing your entire physical body with a shimmer that does not seem to match any known color. When casting a spell that deals elemental damage, roll an additional d10. The roll of this d10 determines an element based on the table below. The spell does additional damage of that element equal to your Constitution modifier.

1 - Acid
2 - Cold
3 - Fire
4 - Force
5 - Lightning
6 - Necrotic
7 - Poison
8 - Psychic
9 - Radiant
10 - Thunder

Virulent Aspect

At 6th level, your Passenger's power begins to bleed into the environment around you. Your appearance changes slightly and you learn to release a concentrated surge of your Passenger's energy into the surrounding area. As a bonus action, you can spend 2 sorcery points to create a 15 foot aura that lasts for 1 minute. This aura follows you as you move and surges according to certain interactions. This aura has different effects dependent upon your Passenger type. 

Parasite. A deep red splotch develops in the center of your chest. Streaks of gangrenous flesh radiate outward from this splotch to your extremities. Whenever you cast a spell that causes damage, any creature in your aura, including yourself, takes damage equal to half of 1 of the spell's damage die. This damage cannot be reduced. The sum of all damage taken by any creature due to the aura is then added to the spell's overall damage. 

Symbiote. A faint, ethereal glow accompanies your body movements, giving the appearance of a second body following your motions. If an enemy hits you with a ranged or spell attack while outside of your aura, they must reroll to confirm. They must use the new roll. If the spell used requires a saving throw, you have advantage on the save. 

Anomaly. Small tendrils of light of an unknown color cover your body and appear to undulate towards anything that approaches near you, sticking slightly if contact is made. Any living creature that enters your aura notices its body beginning to wither, taking on a strange, grey brittleness. Upon entering your aura, a creature becomes vulnerable to Thunder, Force and Bludgeoning damage and takes 9 (3d6) Radiant damage. Upon exiting your aura, the creature loses this vulnerability and takes 18 (6d6) Necrotic damage. This does not affect Constructs. 

Renewed Sovereignty

Starting at 14th level, your body has become inured to the potent force of your Passenger, allowing you to exert some modicum of control over its influence. The manifestation of this depends on your Passenger type. 

Parasite. You may choose whether or not any creature, including yourself, is affected by the surge of your Virulent Aspect aura. Additionally, the effect of your Mortal Conduit feature now causes additional damage equal to the maximum of half of the spell's damage die, rounded down.

Symbiote. The effect of your Virulent Aspect aura now also benefits any allies within it. Additionally, you now have advantage on all Perception and Investigation checks, as well as Wisdom saving throws and Constitution saving throws to maintain concentration on a spell.

Anomaly. You may choose whether or not any creature is affected by your Virulent Aspect aura, and your aura now affects Constructs. Additionally, whenever you take damage from a melee or melee spell attack while your aura is active, the aura surges, dealing 9 (3d6) Radiant damage and 18 (6d6) Necrotic damage to creatures of your choice within it.

True Nexus

At 18th level, your connection with your Passenger has become total, in mind, body and spirit, allowing you to tap into the deepest wells of your Passenger's power without being overwhelmed. Your appearance changes drastically and you are capable of ability beyond what you could previously do. These changes depend upon your Passenger type.

Parasite. Several differently pupiled globules become visible beneath the skin of your forehead. They cause the skin to bulge and ripple perilously as they frantically glance about. Your skin takes on a pallid hue and veins of black blood radiate out from your eye sockets. You may expend 4 sorcery points and 18 (6d6) HP to guarantee a Critical Hit on any spell that causes damage. 

Symbiote. A large glowing eye appears in the center of your forehead, and instills a feeling of harmony and clarity in those who meet its gaze. Your body becomes sheathed in a subtle glow that mimics your form and movements. You may exchange 5 sorcery points for a Legendary Resistance, allowing you to automatically succeed a saving throw, or automatically avoid a melee or spell attack. 

Anomaly. Your eyes become deep wells of black through which stars and the depths of interstellar space can be seen. The multitude of tendrils covering your body become tipped with flame-like radiance of an unknown color and the air around you shifts like ripples on the surface of water. When you hit a creature with a spell that does elemental damage, you may expend 6 sorcery points to infect it with strange radiation. The affected creature sheds light of an unknown color in a 10 foot cube, cannot become invisible by any effect, and at the start of each of its turns takes 18 (6d6) Radiant damage, 18 (6d6) Necrotic damage, and 18 (6d6) Poison damage. A successful Constitution save against your spell save DC ends this effect.