Such activities occur in a liminal space on the edge of the officially sanctioned Theravādin praxis. Buddhist myths also feature heavenly beings, called devas. Buddhist myth adopted several Indian figures such as Brahma, Indra (also known as Sakka) and Prithvi. The schools of Buddhism told stories of the origin of their own particular school. These narratives function like creation myths, explaining how the school came to be, and why it has a special authority to convey the Buddha's teaching. Unlike the pan-sectarian myths of the Buddha's life or the Jātakas, these exist specifically to promote one's own school in relation to contemporary rivals. Such sectarian myths also typically include an account of how the Dharma triumphed over primitive and violent religious cults, especially human or animal sacrifice in worship of yakkhas. The Theravāda origin story is found in multiple places, such as the Dīpavaṁsa, where the Buddha himself is said to have predicted the spread of Theravāda to Sri Lanka. Moreover, in both the Dīpavaṁsa (Island Chronicle) and the Mahāvaṃsa (Great Chronicle), the Buddha is said to have actually visited the island three times. The Sarvāstivādins located their origins in the time of Ashoka, claiming to be the true Dharma that spread to Kashmir when Buddhism in the Middle Country had become corrupt. Other stories developed to give authority to certain texts. The Mahāyānists needed to address the fact that their texts were unknown in the initial period of Buddhism, and developed stories such as that they had been hidden in the realm of the nagas (snake-like supernatural beings) until people wise enough to understand them were born. Some versions of the myth state that the Indian philosopher Nagarjuna magically flew to the city of the nagas and retrieved the hidden sutras. Other myths deal with Buddhas in other worlds which can be reached through dreams or meditative visions such as Maitreya or Amitabha and who reveal new texts and teachings such as the five treatises of Maitreya. Later Mahayana Buddhists also wrote their own biographies of the Buddha which included Mahayana elements, such as the biography in the Tibetan Bu-ston's (1290–1364) "Chos ’byung" (“History of Buddhism”). Similarly, the promoters of the Theravādin Abhidhamma claimed that Abhidhamma had been taught by the Buddha to his mother in Tusita heaven.