The Primal Vow also known as the 18th Vow is part of a series of 48 vows that Amitabha Buddha made in the Larger Sutra of Immeasurable Life. The text of the Primal Vow according to Larger Sutra reads:
Amitabha

Amitabha

Amitābha is a celestial buddha described in the scriptures of the Mahāyāna school of Buddhism...

 Buddha is often associated with devotional practices in Buddhism, as he strives to save those beings who are incapable of reaching Enlightenment through their negative karma, by leading them to Enlightenment. The founder of Jodo Shu

Jodo Shu

, also known as Jōdo Buddhism, is a branch of Pure Land Buddhism derived from the teachings of the Japanese ex-Tendai monk Hōnen. It was established in 1175 and is the most widely practiced branch of Buddhism in Japan, along with Jōdo Shinshū....

 Buddhism, Hōnen, emphasized the importance of the Primal Vow over the efficacy of practices espoused by his contemporaries in Tendai

Tendai

is a Japanese school of Mahayana Buddhism, a descendant of the Chinese Tiantai or Lotus Sutra school.Chappell frames the relevance of Tendai for a universal Buddhism:- History :...

 Buddhism. These same teachings became central to the later Jodo Shinshu

Jodo Shinshu

, also known as Shin Buddhism, is a school of Pure Land Buddhism. It was founded by the former Tendai Japanese monk Shinran. Today, Shin Buddhism is considered the most widely practiced branch of Buddhism in Japan.-Shinran :...

 Buddhist sect as well.

This vow forms the basis of Pure Land

Pure land

A pure land, in Mahayana Buddhism, is the celestial realm or pure abode of a Buddha or Bodhisattva. The various traditions that focus on Pure Lands have been given the nomenclature Pure Land Buddhism. Pure lands are also evident in the literature and traditions of Taoism and Bön.The notion of 'pure...

 Buddhism as well as the practice known as nianfo

Nianfo

Nianfo , is a term commonly seen in the Pure Land school of Mahāyāna Buddhism...

 (Ch.), or nembutsu (Jp.) in Asia.