The nectar obtained from the samudra manthan, done by the devas and asuras. As per Agnipurana Chapter 152, Mrit is a liquid obtained from begging and Amrit and Pramrit are those received without begging.
Once when Durvasa, an ansh incarnation of Shiva was traveling in the forest, he saw Menaka, an apsara with a garland of Kalpavriksha flowers. Its scent filled the forest. Durvasa requested for the garland to be given to him, and wearing it on his head, he reached devaloka. There, he saw Indra on the Airavata along with the devas. He gifted the nectar filled garland, surrounded by bees to Indra, who kept it on top of Airavata’s head. The elephant took the garland in its trunk, smelled it, and flung it to the ground. Durvasa thought Indra had insulted him and cursed that devaloka would lose its prosperity. Indra was stunned and begged for forgiveness. Durvasa said that he was not the kind of sage who forgave and that Indra had become this vain because rishis like Vasishta and Gautama had praised him. Durvasa left and Indra returned to Amaravati. From then, devaloka started losing its lustre, yagas ceased to happen and the devas began to age. Seeing this, the asuras planned to attack the devas, and when they did, the devas were helpless. They approached Brahma, who directed them to Vishnu. The devas started praying to Vishnu, who appeared and promised to help them. He asked to mix the best medicinal herbs in the ocean and churn it using the mountain Meru, and Vasuki as the rope. He told them that when they drank the amrit that would emerge, they would regain their abilities. He also said that he’d ensure the asuras wouldn’t get a share.
The devas made peace with the asuras and prepared for the samudra manthan. Together they collected the herbs, mixed them in the foamy white ocean, and started churning it using Vasuki and Meru. The devas caught Vasuki’s tail while the asuras had the head. The poisonous air coming out of Vasuki’s mouth made the asuras weak, but showers at the tail end invigorated the devas. Vishnu took the form of a giant tortoise and served as the base for Meru. He also simultaneously took another form invisible to both the devas and asuras and pressed the mountain from above.
The first to emerge from the churning was Kamadhenu. As both the asuras and devas watched transfixed, Varunidevi emerged, followed by parijata. Then came the apsaras, followed by Chandra, who was accepted by Shiva. Sixth came the poison which was absorbed by the nagas. Then appeared Dhanwantari with the vessel filled with amrit. This made everyone happy. Lakshmi then emerged seated on a lotus. The gandharvas sang before her, and the apsaras danced. Ganga arrived there with her tributaries to serve as a bathing ground for Lakshmi. The ocean itself presented her a garland of flowers which would never wither. Brahma gifted her jewelery and as everyone watched, she took refuge in Vishnu. This irked the asuras, they seized the amrit from Dhanwantari and ran away.
The devas were bemused by this and made plans to get back the amrit. Vishnu transformed into a beautiful woman called Mohini, and approached the asuras. The asuras asked her who she was. Mohini replied that she was Dhanwantari’s younger sister, and by the time she had emerged from the ocean, everyone had left, and that she was now searching for suitable groom. The asuras told her that she should distribute the amrit among all of them and then choose one of them as a husband. Mohini agreed and said that the asuras should close their eyes, and she would serve them the amrit. Whoever opened their eyes last would serve her the amrit and also marry her. Accordingly, the asuras closed their eyes, and Mohini took back the amrit to devaloka.
When the asuras opened their eyes, Mohini had disappeared. They understood their folly and pursued Mohini. The devas had appointed Surya and Chandra to guard the gates. As per the asuras’ instructions, Rahu disguised himself and tried to enter devaloka. He was discovered by Surya and Chandra and Vishnu injured him with his chakra. Rahu swore revenge at Chandra and Surya. The Bhagavata Skanda 8 explains that this is why Rahu frequently swallows Surya and Chandra and they escape through the cut at the neck. This is the cause of Suryagrahan and Chandragrahan.
Indra and the other devas consumed amrit and were soon attacked by the asuras. The energised devas routed the asuras and scattered them in different directions, and the world regained its prosperity.
As per Mahabharata Adiparva Chapter 18 verses 42-45, after all the good entities had emerged from the samudra manthan, the poisonous kalakuta made its appearance in the form of smoke and flames. It was so strong that everyone fell unconscious. Brahma feared that the world might be in peril and asked Shiva to consume it. Shiva drank it and made it stay in his throat. From then on, he was also known as Neelkanth.
The reason for samudra manthan was Airavata. But as per Mahabharata Chapter 18 verse 42, Airavata, flawlessly white and with four tusks emerged from the manthan and was taken by Indra. Also, in Ramayana Aranyakanda Sarga 14, when an injured Jatayu recites his antecedents to Rama, he mentions the origin of Airavata thus – Kashyapa married the eight daughters of Prajapati Daksha. One of them was Krodhavasha and their children were Mrigi, Mrigamada, Hari, Bhadramada, Matangi, Sharduli, Shweta, Surabhi, Surasa and Kadru. Bhadramada had a daughter called Iravati. Airavata was the son of Iravati. The dissonance is explained partially in Vishnupurana Ansh 1 Chapter 3. Six manvantaras have been completed now. Each manvantara has several Indras. Therefore it is possible that each of them had a different Airavata.
As per Mahabharata Adiparva Chapter 27, Kashyapa’s wives Vinata and Kadru gave birth to Garuda and the nagas respectively. Once Vinata said that the tail hairs of Indra’s horse Uchaishravas were all white, Kadru maintained that it was black. It was agreed that whoever lost would become the other’s slave. As per Kadru’s instructions, a few nagas clung to the horse’s tail and made it appear black. Thus Vinata became Kadru’s slave and Garuda had to take care of Kadru’s children. Kadru said that she would end the slavery if Garuda got the amrit from devaloka. Garuda defeated the devas and got the amrit. The nagas kept the amrit on darbha grass and went for a bath. During that time, the devas took back the amrit. When the nagas returned, they couldn’t find the amrit, but they were so filled with lust for it that they began licking the grass. Their tongues were torn and that’s the reason why snakes have a forked tongue. As per Mahabharata Adiparva Chapter 17, Chapter 27 verse 16, Chapter 30 verse 2, Ramayana Aranyakanda Sarga 35, Vishnupurana Ansh 1 Chapter 9, Agnipurana Chapter 152, Bhagavata Skanda 3 and Uttararamayana, amrit is now safely stored in devaloka.