Upon Sun Wukong achieving immortality, his Buddho-Daoist master Subodhi warns him of three calamities sent by heaven to kill those who defy their fate and attain eternal life. The Sage then offers to teach Monkey one of two forms of transformation in order to avoid this outcome by living in hiding.  The first form, called the “Multitude of the Heavenly Ladle” (Tiangang shu, 天罡數), contains thirty-six changes, while the second, the “Multitude of Earthly Fiends” (Disha shu, 地煞數), contains seventy-two.  Our hero chooses the latter and quickly masters a set of secret oral formulas (koujue, 口訣). This becomes one of his signature abilities used throughout the narrative. Monkey’s most famous use of the skill appears in chapter six when he battles Lord Erlang, a divine demon queller and fellow master of transformations (video 1).
Video 1 – Sun’s battle with Lord Erlang. From the great animated Classic Havoc in Heaven (1965).
I. Connection to Chinese astrology and literature
The names of the two forms of transformation that Subodhi offers to teach Monkey can be traced to Chinese astrology. The “Heavenly Ladle” (Tiangang, 天罡; i.e. the Big Dipper) is associated in some traditions with thirty-six stars (fig. 1). Regarding the origin of these stellar bodies, Werner (1932/1969) explains: “The gods of these stars (all stars of good omen) are all heroes who fell on the field of battle in the epic combat known as Wan Xian Zhen 萬仙陣, “The Battle of the Myriad Genii [or Immortals]” (p. 506). 
Fig. 1 – A list of the thirty-six Heavenly Ladle stars (larger version). Photograph of Werner, 1932/1969, p. 506. Apologies for not having access to a scanner at this time.
Furthermore, he writes that the “Earthly Fiends” (Disha, 地煞) are:
[S]eventy-two stars [fig. 2] of evil influence, opposed to the Tiangang. The wicked genii of these stars are cast out and slain by tongzi 童子 magicians [i.e. spirit mediums], who impale them on forks and shut them up in earthen jars, then take them to waste lands, throw them into fires, and surround the spot with a circle of lime, which is supposed to prevent any spirit which may have survived the burning from getting out of it (Werner, 1932/1969, p. 496). 
Fig. 2 – A list of the seventy-two Earthly Fiend stars (larger version). Photographs of Werner, 1932/1969, pp. 496-497.
Additionally, the Earthly Fiends are considered the “enemies of man, and causes of all diseases and ailments” (Doré & Kennelly, 1916, p. xviii). Several Buddho-Daoist folk talismans exist to ward afflictions caused by the Fiends. One such Buddhist talisman said to cure the “one hundred ailments” even invokes the thirty-six Heavenly Ladle stars to aid in the conquering of the seventy-two demons:
An order is hereby made by the “Ministry of the Thunderbolt”, commanding in the name of the “three religions” that the auspicious stellar gods, Tiangang 天罡, reduce to order the maleficent demons, Disha 地煞, who have caused this disease. The charm must also repress these malignant beings and expel them forthwith (fig.3) (Doré & Kennelly, 1916, p. 312).
Fig. 3 – A reproduction of the illness-curing Buddhist Talisman (larger version).
It’s interesting that Sun Wukong chooses the transformation method centered around stars of evil influence and later becomes a demon who challenges heaven.  Good fodder for fan fiction, no?
When these dichotomous stellar bodies were first acknowledged isn’t exactly clear.  But the Heavenly Ladle stars go back to at least the mid-13th-century as they are mentioned in the Old Incidents in the Xuanhe period of the Great Song Dynasty (Da Song Xuanhe Yishi, 大宋宣和遺事) (Anonymous, n.d.), a storytelling prompt of the late-Song to early-Yuan. It contains the earliest stories associated with the Water Margin (Shuihu zhuan, 水滸傳, c. 1400), a Chinese classic that predates Journey to the West. The one hundred and eight heroes of this novel are famous for being reincarnations of the Heavenly Ladle and Earthly Fiend stars, a fact revealed in chapter seventy-one when a heaven-sent stone slab is found to list their human names along with the corresponding stellar titles. The long association of the stars with the hugely popular Water Margin novel therefore may have inspired the names for the techniques taught by the sage Subodhi in Journey to the West.
II. Ties to Daoist practices
Robinet (1979) expertly explains that transformation (bianhua, 變化), or “metamorphosis” as she calls it, is central to Daoism. Gods and Saints are portrayed in Daoist literature as being in constant flux, changing with the seasons, taking on different guises and titles, disappearing and reappearing, never remaining the same, thereby living eternally. Daoists and magicians achieve metamorphosis through external and internal alchemical processes, the former involving the ingestion of drugs and talismans and the latter via mental exercises. Those who succeed in their practice can divide themselves endlessly; create rivers, mountains, and forests from meager samples of water, earth, and seeds; and, most importantly, change their form into anything (fig. 4), including the five elements, dragons, clouds, rays of light, or even celestial bodies like the sun and moon.
Fig. 4 – The cover of a vintage children’s flip book about Monkey’s transformations (larger version). Here he is seen changing into a fish.
Interestingly, transformations could be used to live in hiding, much like originally intended by Subodhi in Journey to the West. Adepts still questing for immortality could magically transform a sword, staff, or slipper into their deceased body, thereby faking death and escaping elsewhere to find a method leading to eternal life. (Often times, those who took this route assumed a new identity to avoid heaven’s gaze (Campany, 2005)). Additionally, sages are said to use their powers to hide in the earth or in the light of the sun, moon, and stars. One source mentions adepts hiding by scattering their shadow and transforming it into seventy-two types of light. In a related book chapter, Robinet (1993) notes this number “alludes to [Laozi’s] seventy-two supernatural marks” (clearly borrowing from the Buddhist Mahapurusa laksana) (p. 166). This is fascinating as it shows there is precedent for seventy-two transformations in Daoism.
III. Archive link
I have archived Robinet’s (1979) wonderful paper on metamorphosis. It can be read here:
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1) It should be noted that the calamities are sent every five hundred years. Sun never has to live in hiding, though, as he is trapped under Five Elements Mountain upon the five hundredth anniversary of his immortality (he lived to be roughly four hundred prior to taking up spiritual cultivation). And he achieves Buddhahood prior to reaching the one thousandth year of his immortality, so he never has to guard against subsequent calamities.
2) The translation of these names are loosely based on Anthony C. Yu (Wu & Yu, 2012, vol. 1, p. 122). I have provided more accurate names based on related Chinese literature (see section one above).
3) Source changed slightly. I updated the Wade-Giles to Pinyin. This refers to a military trap appearing in the Chinese classic Investiture of the Gods (Fengshen yanyi, 封神演義, 16th-century), which was published around the same time as Journey to the West.
4) I’ve previously mentioned a similar ritual in the first section of this article.
5) Conversely, Zhu Bajie is shown capable of thirty-six transformations (for example, Wu & Yu, 2012, vol 2, p. 328), meaning he studied the method associated with the stars of good omens. And of course we know his sordid story…
6) Though, in my opinion, the thirty-six stars are likely based on the thirty-six generals led by the stellar exorcist, Marshal Tianpeng (天蓬, i.e. Zhu Bajie’s former incarnation), who is himself one of the nine stars of the Big Dipper. The Marshal and his generals appear in the liturgy of the Song-era “Correct Method of the Celestial Heart” (Tianxin zhengfa, 天心正法) exorcist tradition (Anderson, 2008).
Anderson, P. (2008) Tianxin zhengfa In F. Pregadio (ed.), The encyclopedia of Taoism: Vol 1-2 (pp. 989-993). Longdon: Routledge.
Anonymous. (n.d.). Da Song Xuanhe Yishi [Old incidents in the Xuanhe period of the Great Song Dynasty]. Retrieved from https://ctext.org/wiki.pl?if=gb&res=290731&searchu=%E5%A4%A9%E7%BD%A1
Campany, R. (2005). Living off the Books: Fifty Ways to Dodge Ming 命 [Preallotted Lifespan] in Early Medieval China In C. Lupke (Ed.), The Magnitude of Ming: Command, Allotment, and Fate in Chinese Culture (pp. 129-150). University of Hawaii Press.
Doré, H., & Kennelly, M. (1916). Researches into Chinese superstitions: Vol. 3 – Superstitious practices. Shanghai: T’usewei Printing Press. Retrieved from https://ia800709.us.archive.org/2/items/researchesintoch03dor/researchesintoch03dor.pdf
Robinet, I. (1979). Metamorphosis and deliverance from the corpse in Taoism. History of Religions, 19(1), 37-70.
Robinet, I. (1993). Taoist meditation: The Mao-shan tradition of Great Purity. Albany: State University of New York Press.
E. T. C. Werner (1969). A dictionary of Chinese mythology. New York: The Julian Press. (Original work published 1932)
Wu, C., & Yu, A. C. (2012). The journey to the West: Vol. 1-4. Chicago, Illinois: University of Chicago Press.