Vāsuki is a serpent king in Hindu mythology who is described as having a gem called Nagamani on his head. Vāsuki is famous for coiling around Shiva's neck, who blessed and wore him as an ornament.
In the legend of the churning of the milk ocean, Vāsuki allowed the Devas and Asuras to bind him to Mount Mandar to use him to churn the Amrita, the nectar of immortality. This happens after the devas and the asuras agree to join forces in order to gain the Amrita. It is also said that Indra tricked the asuras into holding Vāsuki by its heads, so that they could inhale its hot breath. However, as the milk ocean was churned up, a poison seeped out which threatened the world. So Shiva swallowed the venom as an act of self-sacrifice to prevent premature destruction of the world, thus earning him the title of Nilakantha ("blue throat"). He has a sister called Manasa, who is a goddess of snakes and poison.
Vasuki is also recorded in Buddhist mythology, as well as the mythology of China and Japan, as one of the "Eight Great Naga Kings" (known in Japan as the Hachidai Ryuu-Ou). In Buddhist mythology, he, along with the other Naga kings, appear in the audience of many of the Buddha's sermons.
Vasuki acts as one of the two chaos field bosses tied to the Invoke system and will appear in Ueno once every hour during weeks when the chaos Alignment dominates the system. After obtaining the poison snake hand he drops, he can be fused by a special double-fusion of Raja Naga and Garuda.
Shin Megami Tensei IV
Vasuki is initially the subject of the Challenge Quest "Tokyo Bay Mixer." Flynn must hunt for the demon in the overworld area of Infernal Tokyo. Vasuki can be found in areas surrounding the large pool of poison north of Camp Ichigaya. Two Vasuki Hides are required in order to complete the quest.
Shin Megami Tensei IV Apocalypse
Vasuki can teach Nanashi the Poison Breath, Glacial Blast and Ice Age skills through his Demon Whisper, but cannot pass on the Shivering Taboo skill in any way. Vasuki benefits from learning ailment and Ice skills.
Shin Megami Tensei V
"A giant serpent of Hindu lore. It is said that gods and demons used him as a rope to churn the sea of milk to create Amrita, while also using Mt. Mandara as the stick. The strain from this caused him to exhale incredibly poisonous venom, but it was safely swallowed up by Shiva."
—Shin Megami Tensei V lore
Vasuki serves as Shiva's right-hand demon and his representative in Bethel as the god of destruction is mostly uninterested in such events. He witnesses Abdiel lose to the protagonist and decides to inform him of the events. Vasuki is also entrusted the Key of Austerity and is confronted by the protagonist for it; confident that he would win, Vasuki is surprised when he is bested by the protagonist. He forfeits the key and apologizes to Shiva in his dying breath, which in turn unlocks him for fusion. He requires a special fusion of Ananta, Yurlungur and Fafnir.
"A giant serpent of Hindu legend. It is said the gods and demons used him to churn the sea of milk. The strain caused him to exhale incredibly potent venom, but Shiva swallowed it."
The advanced fusion to produce Vasuki is unlocked at Rank 1 of the StrengthConfidant, which can be started by talking to the twins starting on 5/18. The task they give the protagonist is to show them a Jack Frost with Mabufu, which he learns at level 12. The components required to fuse him are Naga, Raja Naga and Ananta.
Digital Devil Saga: Avatar Tuner
Vasuki appears as the first boss of the Karma Temple and is fought in the Temple's Sea of Milk after the Embryon is kicked to the very bottom of the tower by Sera to prevent them from following her. He acts as a guardian, intended to deter intruders from ascending the Temple. Vasuki fights mainly by exploiting weaknesses and Critical Hits with his unique spell Hima Alaya (an Ice-type spell which deals heavy damage and high chance of inflicting Freeze), Maziodyne and Megidola.
Devil Survivor 2
"A giant serpent of Hindu lore. It is said that gods and demons used him to churn the sea of milk to create Amrita. They used Mt. Mandara as the stick and Vasuki as the rope to pull it. The strain caused him to exhale incredibly poisonous venom, but Shiva safely swallowed it."