The Sanguilith, occasionally referred to as the God of (Meat and) Mouths or the Adversary, was considered by K'thriss to be an 'intercessor' between him and the god he actually seeks to follow, The Ur. After mentally reaching out to an outer-worldly entity using the spell Contact Other Plane, K'thriss became briefly insane and later dropped from level 9 to level 3 with a bout of amnesia. He subsequently discovered that his warlock pact to the Ur had become subverted, and that he was now connected to the Sanguilith, despite still carrying an arcane focus dedicated to the Ur. K'thriss tolerated this arrangement only because the being promised to help K'thiss learn more about his true patron, and K'thriss was confident the Ur was stronger than it and could still protect him.
Background (Season 1 spoilers)
As the C Team returned from a mission and crossed the Sumber Hills, they stumbled upon the Wandering Crypt. Deep inside, K'thriss discovered a Black Gate marked with symbols of different gods. His arcane focus bearing the symbol of the Ur seemed to resonate with it, but was not sufficient to open the gateway, and triggered an acid trap that forced them to flee.
It was not long until the group was actually assigned by the Acq Inc CEO, Omin Dran, to return to the Crypt and retrieve his lost sister. Where it had once been, they instead found scrawlings of the symbol of Maelith--a goddess who Walnut's clan lost faith in and considered a betrayer. They also found an unconscious tiefling and took her along with them, not knowing that she was secretly a follower of that goddess (in actual fact, it was a cult to the Sanguilith) and would later turn against them. K'thriss formed a bond with Maelith and Walnut with Mielikki, and when the two communed, K'thriss discovered that Maelith was imprisoned by the Sanguilith. When they went looking again for the Crypt, Walnut perceived the image of a large maw with rows of amethyst teeth. She used a druidic weapon to pin the Crypt in place and open two entrances. Inside one, they found members of Walnut's enclave petrified in the middle of performing a ritual of binding around the drawing of a buried crystal ('shardling', another name for the Crypt[5,16]). Now with three god symbols in hand (Ur, Oba-ma, Maelith), K'thriss was able to open the original Black Gate, leading to a chamber where he felt the presence of the Ur, but could not communicate with it. Following a spiraling shaft deep into its center, the C Team fought servants of the Sanguilith and encountered a vision of young Omin, promising again to find his sister.
Later, K'thriss discovered yet another black gate at the bottom of a lighthouse inscribed with the familiar god runes. Accompanied by Walnut, they passed through it into the Sanguilith's lair, which was now shaped as an amethyst cathedral, suspended in outer space with an open ceiling aimed at the Homeward Star. It was supported by eight crystal pillars, each inscribed with a glyph and imprisoning a deity. K'thriss found the Ur in the first of those prisons, and the goddess Maelith trapped in another. The Sanguilith sensed the presence of Mielikki inside the druid, and assumed that K'thriss had brought her as an offering. When K'thriss refused, the Sanguilith admonished him for having provided 'scant offerings' in the past, and reminded him that the universe was nothing but 'meat and mouths'. Walnut saw that Maelith was in a pale, partially consumed state, and used her treant form to try to break her out. In return, the Sanguilith used the powers it had stolen from the Ur to change the past and undo Walnut's heritage. K'thriss was later able to use his deeper understanding of the universe to reverse this.
Although K'thriss's connection to the Ur allowed him to summon tentacles of darkness, his familiar was a sentient solitary tentacle empowered by the Sangulith and covered in its amethyst shards, named Ligotti. Ligotti was also opposed to the Sanguilith, and told K'thriss that its goal was to consume its own parent.[7,9] However, it was shown to be susceptible to the will of the Sanguilith, as it was forced to gouge out K'thriss's eyes, a process that replaced the drow's own eyes with crystals. Previously, when K'thriss had used his connection with Ligotti to see through its multifaceted eyes, he had noted it had a form of 'hyper-vision'.
Purpose and Abilities (Spoilers for Seasons 2 & 3)
The Sanguilith is described by the Ur as a 'void child, the divine prison', but was only recently revealed to actually be a construct of pure order created by The Ur to contain the chaos of the god Hastur, requiring the powers of other gods to charge itself.. The Ur had told K'thriss that it had allowed itself to be imprisoned and did not wish to escape, also later elaborating that "the first three [gods] entered willingly" as part of a 'coalition', but that after it consumed Hastur, it became more parasitic and required even more gods to feed it.
The order in which the gods were imprisoned appears to be significant, as the Ur revealed that they can only be released in the reverse order in which they were devoured.[9,10] The order of capture seems to be related to the order in which the pillars are laid out across the cathedral, which is:
That-Which-Endures was released through the efforts of the C Team at the request of its mind flayer servant in the Underdark. Maelith was released through a Vistani wedding ceremony between Walnut and Brahma, using magical rings that also contained Evelyn Marthain's soul. Following the ceremony, the C Team were assaulted by crystalline insectoid monsters sent by the Sanguilith. On several past occasions the Sanguilith had also attacked the party using giant amethyst shards, fired across vast expanses of time and space.
When Coriander consumed enough magical artifacts that it was able to transform itself into a spaceship, the C Team found itself traveling through the Astral Plane and ran into a fleet of halfling ships dedicated to stopping the Adversary. One of the halflings was Rosie's own father, who introduced to Rosie her half-sister, and mentioned that her mother had traveled on ahead to initiate their plans against the Sanguilith. The ships used special technology to make themselves invisible to the Adversary, and Rosie discovered that looking at the enemy through a shard of its own crystal could reveal what it tried to keep hidden--such as the extent of its control over the Homeward Star system. As the ships traveled to the last free world, the planet revealed itself to be a trap, piloted by Rosie's mother, which opened itself like a giant claw to grasp the Sanguilith's amethyst prison. The Sanguilith in turn sent out angelic minions--thought to be servants of Celestian that it had taken over and armed with amethyst weapons. The Sanguilith took hold of Donaar and managed to rip the dragon god Vars Melis out of him, after which Donaar was teleported to safety by the spirit of his brother just as the minions shot at him with an eldritch arrow. Evelyn Marthain also made an appearance, possibly summoned by her god Lathander, but when she tried using her holy powers to smite the angelic minions, it was turned on her own allies instead. It was then hinted that Lathander was also taken over by the Sanguilith, as a new wall of the prison began to glow, as if preparing itself to receive him.
Meanwhile, K'thriss had found that his body was becoming more and more consumed by the Sanguilith's crystals. After the drow's death, Donaar used the power of his god to revive K'thriss's old body, but his soul instead chose to enter a reincarnated female drow body that Walnut had created for them. Their hope was that this new body would have no tie to the Sanguilith, but it began to assert control nevertheless, until K'thriss rejected it outright, and devoted their pact instead to Maelith. Ligotti then began to animate K'thriss's old body and attack them. It restated its intent to destroy the Sanguilith and take its power for itself. Although K'thriss was proud of it in a way, they were disappointed in its cowardice when it found itself outmatched and fled.
The Sanguilith was able to communicate to Rosie through a shared hallucination with her, Evelyn, K'thriss and Walnut, and the uncanny cat Onyx was able to inform them what was real and what wasn't. The Sanguilith used the persona 'Sibil' to represent itself, and showed them a vision of the Ur organizing pieces around a battle map. Despite Sibil acknowledging that what the Ur did was important, she said that these small details wouldn't matter if the enemy managed to break through. Rosie and K'thriss then tried to persuade the others that the Sanguilith was a necessary evil and that they needed to feed it more gods to keep Hastur imprisoned. Walnut instead urged her goddess to flee, and Donaar didn't understand that his god was there for a purpose, and continued to try to free him. As the Sanguilith began to crack, K'thriss focused his thoughts on drawing Hastur away by directing it towards a more ordered universe. Per Jerry, the walls of Hastur's prison shrank and disappeared "as the four cleaver-like blades of Pleasant Tomenook's world ship collapse in, crushing the structure completely... Having freed no less than six gods, the landing [the C Team] receive on the strange surface of the last planet in the Homeward Star system is very soft indeed".
 C Team S1E15, The House Wins Part 1
 PAX East 2017, Acquisitions Incorporated
 C Team S1E8, Doomgate Inn Part 3
 C Team S1E9, Homecoming Part 1
 C Team S1E23, Anchors Away Part 1
 C Team S1E31, Dran Incorporated Part 1
 C Team S1E26, Anchors Away Part 4
 C Team S2E3, Plans within Plans, Part 2
 C Team S3E24, The Velvet Cloak, Part 10
 Series Rewatch Notes Season 2. C Team Reddit
 What is the God Order? C Team Reddit:
 What we know of the Eight Gods. CTeamUpdates tumblr.
 C Team S2E11 , Plans Within Plans Part 10
 C Team S2E34 , Matriarch Part 8
 C Team S3E23, The Velvet Cloak, Part 9
 The Ecology of the Wandering Crypt. Twitter.com.