The eight immortals (Chiranjeevis) in Hindu scriptures are Ashwatthama, Kripacharya, Veda Vyasa, Vibhishana, Lord Parashurama, Lord Hanuman, King Mahabali, and Sage Markandeya. There is some confusion over the number of immortals, whether there are seven or eight Chiranjeevis, but a mantra lists the names of all eight immortals.
It is as follows:
अश्वत्थामा बलिव्र्यासो हनूमांश्च विभीषण:।
कृप: परशुरामश्च सप्तएतै चिरजीविन:॥
सप्तैतान् संस्मरेन्नित्यं मार्कण्डेयमथाष्टमम्।
जीवेद्वर्षशतं सोपि सर्वव्याधिविवर्जित॥
Transliteration:
Ashwathaama Balirvyaaso Hanumanshcha Vibheeshanaha
Krupaha Parshuramascha Saptaitey Chiranjivinaha
Saptaitaan Samsmareynnityam Markandeymathaashtamam
Jivedvarshshatam Sopi Sarvavyadhivivarjit
Meaning: The one who remembers these seven immortals daily, along with Markandeya, the eighth, lives for 100 years without any ailments.
The Eight Immortals in the Hindu Scriptures:
1. Ashwatthama:
Ashwattama is the son of Dronacharya, who was the teacher of the Kauravas and Pandavas. He played a vital role in the Mahabharata. He is an incarnation of Lord Shiva.
There are two streams of thought behind how Ashwatthama became immortal.
A. Ashwatthama got his immortality through a curse by Lord Krishna. After the Mahabharata War was over, he deceitfully killed the five sons of Pandavas and Draupadi, along with many other soldiers. When the Pandavas found and tried to kill him, he invoked Brahmastra. Arjuna also did the same. Foreseeing the destruction of nature by the weapon, Lord Krishna intervened and ordered them to take back their weapons. Arjuna followed his command, but Ashwatthama did not know how to take it back. So, he launched it upon the womb of pregnant Uttara, killing her unborn son.
Lord Krishna lost his temper after seeing his brutality and removed the jewel from his forehead, which caused a non-healing wound. He cursed him that he would roam the Earth until the end of Kali Yuga with his festering wound.
B. The second theory is that he was immortal by birth, as he was an incarnation of Lord Shiva. The incarnations of Lord Shiva are immortal unless killed by Lord Shiva himself. Lord Krishna knew he was immortal, so he just removed the jewel from his forehead and made him suffer.
2. Lord Hanuman:
Lord Hanuman is a demigod who is the son of the god Vayu (wind). He is an incarnation of Lord Shiva. Bajarang Bali helped Lord Rama kill Ravana and bring back Goddess Sita to Ayodhya. He is the greatest devotee of Lord Rama. When Lord Rama was about to give up his earthly life by entering the Sarayu river, he gave him the boon of immortality.
Per another story, when Lord Hanuman was a kid, he tried to swallow the Sun as he thought it was a fruit. Indra injured him with the Vajra to prevent him from doing so. It angered the Wind deity, and he started sucking all the air in the world, resulting in the annihilation of life on the Earth. Then the other gods intervened and consoled him by bestowing different boons to Lord Hanuman. Lord Brahma gave him a boon that no divine weapon would harm him, while Indra gave him a boon to choose the moment of his death. Lord Yama blessed him with good health and immortality.
3. Lord Parashurama:
Lord Parashurama is the sixth incarnation of Lord Vishnu. He is the son of Renuka and Sage Jamadagni. He freed the Earth 21 times from arrogant Kshatriyas abusing their powers. Parashurama also played an essential role in the Mahabharata by giving military training to Bhishma and Karna. He was a Brahmin by birth but a warrior by deeds. Lord Shiva gave him military training. During the initiation ceremony, he gave him his amsha (a small part of divine essence). Therefore, he became immortal.
4. Veda Vyasa:
Veda Vyasa, aka Krishna Dvaipayana, wrote the Mahabharata, divided the Vedas into four parts, and wrote the original draft of the Puranas. He is the son of Satyavati and Sage Parashara. He ensured that the Vedas remained safe after the end of Dvapara Yuga and also played a vital role in the Mahabharata. It seems he was immortal since birth. As soon as he was born, he became a full adult male.
5. Kripacharya:

Kripacharya was the great-grandson of Sage Gautama. He was the son of Shardwana and Janapadi. His sister married Dronacharya. Therefore, he was the maternal uncle of Ashwatthama. He was the teacher of Kauravas and Pandavas and also taught warfare to Parikshita, the grandson of Arjuna. Lord Krishna granted him immortality because of his virtues like impartiality, truthfulness, and righteousness.
6. King Mahabali:
King Mahabali is considered the greatest of all Asura kings. He had conquered the three worlds and was the grandson of Prahlada. He lives in Patal Loka (netherworld) and comes to the Earth once a year. King Mahabali would be Indra in the eighth Manvantara.
During the Ashvamedha Yajna, Lord Vishnu in the form of Vamana asked him for three steps of land, which Mahabali granted. Vamana assumed a gigantic form and occupied heaven in the first step and the Earth in the second. As there was no place to put the third step, Mahabali offered the space on his head to Vamana. Lord Vishnu pushed him into the netherworld, but because of his generosity, made him a king of it. He granted him immortality and allowed him to come to Earth once a year. He also blessed him to become Indra in the 8th Manvantara.
7. Vibhishana:
Vibhishana, Ravana’s brother, played a vital role in defeating him. When Ravana abducted Sita, the wife of Lord Rama, Vibhishana advised him to return her to her husband, but Ravana insulted and banished him from Lanka. Therefore, he met Lord Rama and helped him to defeat and kill Ravana. For the protection of dharma, he went against his own brother. Therefore, Lord Rama made him the king of Lanka and conferred immortality upon him. He married Mandodari, Ravana’s wife, per Lord Rama’s instructions.
8. Sage Markandeya:
Sage Markandeya is the son of Sage Mrikandu and Marudmati. They propitiated Lord Shiva and asked for a son. They were given two options, viz. Either they would have a righteous son with a short lifespan or a son with low intelligence, with a long lifespan. They chose the first option.
Therefore, Markandeya was destined to die as soon as he became 16 years old. He was a great devotee of Lord Shiva and continued to worship the Shiva lingam on the day of his impending death. He composed the Maha Mrityunjaya Mantra and continued chanting it. Lord Yama came there and threw his noose around Markandeya’s neck, but it accidentally landed around the Shiva lingam as Markandeya had embraced the Shiva lingam. This act of Yama infuriated Lord Shiva. He came out of the Shiva lingam and defeated Lord Yama in the battle to the point of death. He afterwards revived him on the condition that Markandeya would live forever. Thus, Sage Markandeya became immortal.
Other immortals:
Other than the eight immortals mentioned above, many other personalities in Hinduism are believed to be immortals. Some of them are as follows:
1. Kakbhushundi.
2. Jambavan.
3. Devapi.
4. Saptarishi.
Where are the Eight Immortals in the Hindu Scriptures Currently?
Most of these immortals are invisible to the general public and live in subtle bodies.
1. Lord Hanuman – Gandha Madan Parvat.
2. Mahabali – Netherworld.
3. Lord Parashurama – Mahendra Parvat.
4. Ashwatthama roams in Indian forests. Many claim they have seen him during the Narmada Parikrama asking for oil to apply to his wounds.
5. Veda Vyasa – Haridwar, Vrindavan, and Chitrakut.
6. Vibhishana – it is believed that he visits Jagannath Temple in Odisha daily.
7. Kripacharya – Himachal valley.
8. Markandeya – Unknown.
What Would Happen to the Eight Immortals in the Hindu Scriptures at the End of Kali Yuga?
There are different views about what would happen with these chiranjeevis at the end of Kali Yuga. Some believe they would continue to live until the end of the Kalpa.
As mentioned in the Vishnu Purana, Veda Vyasa, Kripacharya, and Parashurama will become the three sages of the Saptarishis in the 8th Manvantara. Ashwatthama would be the Veda Vyasa in the 29th Dwapara Yuga, King Bali would be Indra in the 8th Manavantara, and Lord Hanuman would be the next Brahma.
There is no information available about Vibhishana and Sage Markandeya.
Please let us know through the comments section if you know anything more about the eight immortals in the Hindu scriptures.
References:
https://en.wikipedia.org/wiki/Markandeya