The Symbiont

You have made a pact with a Symbiont, a rare breed of parasite that requires an intelligent host to exert its cryptic will on the world. Performing a bonding ritual with the creature grafts the Symbiont to you, permanently altering your physiology and mental processes. This allows you to perform eldritch feats of savagery and cunning previously thought impossible. But you’re awfully hungry...

Few have seen a symbiont unattached to a host, and even fewer know what their purpose on the material plane might be. Naturalists theorize that they are oozes who have been warped by negative energy runoff from the Shadowfell, found in certain regions of the Underdark. Though strange and unknowable they may be, most Symbionts display a warped sense of decorum in bonding with a host. If a host is able to consent to the union, the symbiont will require this before initiating the bonding process. It has no scruples about how this consent is obtained, however. This Warlock Patron differs from the other options available to an enterprising young Warlock because the Symbiont is constantly aware of the Warlock’s actions. Magical and non-magical interventions can be used to dull the Symbiont’s senses, but breaking its hold on the host is nigh on impossible. Some Warlock’s even come to depend on their Symbiont for guidance, relying on it to make decisions too difficult or distasteful for them to stomach.

Expanded Spell List

The Great Old One lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Great Old One Expanded Spells

Spell Level	Spells
1st	absorb elements, entangle
2nd	alter self, enhance ability
3rd	animate dead, haste
4th	greater invisibility, Evard’s black tentacles
5th	contagion, mislead

Biomimetic Beacon

Starting at 1st level your Symbiote grants you the skills of a master hunter. You have advantage on Wisdom (Perception) checks that rely on smell. Additionally you may mark a creature by implanting a microscopic piece of the symbiote under its skin. Make a melee spell attack against a target within 30 feet. If the target cannot see you, it is unaware of this attack. On a hit the creature is marked. While the creature is marked in this way, when you hit it with a spell attack you may deal additional acid damage equal to your Charisma modifier (minimum of 1). While the marked target is within 300 feet of you, you know its exact location. You may mark only one target at a time, and the effect ends after an hour when the implant breaks down. 

Eyes in the Back of Our Head

At 6th level your attunement with the your Symbiont gives you an unnatural vigilance. You gain advantage on initiative rolls.

Additionally, if you are surprised at the beginning of combat and aren’t incapacitated, you may spend a warlock spell slot to act normally on your first turn.

Once you use this feature, you can’t use it again until you finish a short or long rest.

Our Blood is a Fortress

Starting at 10th level, you are immune to poison and disease. Additionally you cannot be frightened by aberrations or monstrosities.

We Hunger

At 14th level, you can attempt to nourish the Symbiont with the flesh of your enemies. When you hit a medium or smaller creature with a melee attack, you can use this feature to instantly swallow it.

The swallowed target is Blinded and Restrained, it has total cover against attacks and other effects outside of your body, and it takes 3d6 acid damage at the start of each of the your turns. You gain temporary hit points equal to half the damage done in this way. The creature may attempt to escape the grapple (escape DC 16) at the start of each of its turns.
If you take greater than 20 damage in one turn you must make a DC 15 constitution saving throw. On a failure a swallowed creature is no longer Restrained by you and can escape your gaping maw using 5 feet of Movement, exiting prone.

Once you use this feature, you can’t use it again until you finish a long rest.