Huángbò Xīyùn (simplified Chinese: 黄檗希运; traditional Chinese: 黄檗希運; Wade–Giles: Huang-po Hsi-yün; lit.: 'Xiyun of Mt. Huangbo', Japanese: Ōbaku Kiun) (died 850[a]) was an influential Chinese master of Zen Buddhism during the Tang Dynasty.
Very little about Huángbò's life is known for certain as, unlike other Transmission of the Lamp literature, there is no biographical information included with Huángbò's collection of sayings and sermons, the Ch’uan-hsin Fa-yao (Essentials of Mind Transmission) and the Wan-ling Lu (Record of Wan-ling: Japanese: Enryōroku).
Huángbò began his monastic life on Mt. Huangbo in Fujian province, receiving the Buddhist name Hsi-yun. As was the custom of the times, he traveled around seeking instructions from various Chan masters. He visited Mt. Tiantai and sought teachings from the National Teacher Nanyang Huizhong (Wade–Giles: Nan-yang Hui-chung; Japanese: Nan’yō Echū). At some point he may also have studied under Nanquan Puyuan (748-835) (Wade–Giles: Nan-ch’üan P’u-yüan; Japanese: Nansen Fugan), a student of Mazu Daoyi (Wade–Giles: Ma-tsu Tao-i; Japanese: Baso Dōitsu) (709-788)
However, Huángbò's main teacher was Baizhang Huaihai (Wade–Giles: Pai-chang Huai-hai; Japanese: Hyakujo Ekai), another Mazu student, and it was from Baizhang that Huángbò received Dharma transmission. According to Yuanwu Keqin's commentary in The Blue Cliff Record, when Huángbò first met Baizhang, Baizhang exclaimed, “Magnificent! Imposing! Where have you come from?” Huángbò replied, “Magnificent and imposing, I’ve come from the mountains.” 
In 842, a prominent government official in Kiangsi province, Pei Xiu (Wade–Giles: P’ei Hsiu) (787 or 797-860), invited Huángbò to take up residence at Lung-hsing Monastery. Pei was an ardent student of Chan and received teachings from Huángbò, eventually building a monastery for Huángbò around 846, which the master named Huang-po after the mountain where he had been a novice monk.[c]
Before Huángbò died, he named thirteen successors, the most prominent of which was Linji Yixuan. He was given the posthumous title (probably under the urging of Pei Xiu who became chief minister of the central government in 853) of “Chan Master Without Limits” (Tuan Chi Ch’an Shih).
John Blofeld says he died on Mount Huangbo during the T'ai Chung reign of the Tang Dynasty, or between 847–859. Blofeld says his memorial pagoda is "The Tower of Spacious Karma" and that it was Emperor Hsüan Tsung who gave him the title "The Zen Master Who Destroys All Limitations".
What is known of Huángbò's teachings comes from two texts, the Ch’uan-hsin Fa-yao (Essential of Mind Transmission) and the Wan-ling Lu (Record of Wan-ling: Japanese: Enryōroku) written by Huángbò's student, Pei Xiu.[d] Pei compiled the teachings from his own notes and sent the manuscript to the senior monks on Mount Huangbo for further editing and emendation.
The “official” version of the Huángbò literature was published as part of the Transmission of the Lamp, Compiled during the Ching-te Period, in 1004. The record of Huángbò is more or less equally split between sermons by the master and question and answer dialogues between the master and his disciples and lay people.
Huángbò's teaching centered on the concept of “mind” (Chinese: hsin), a central issue for Buddhism in China for the previous two centuries or more. He taught that mind cannot be sought by the mind. One of his most important sayings was “mind is the Buddha”. He said:
All the Buddhas and all sentient beings are nothing but the One Mind, beside which nothing exists. The One Mind alone is the Buddha, and there is no distinction between the Buddha and sentient beings.
To awaken suddenly to the fact that your own Mind is the Buddha, that there is nothing to be attained or a single action to be performed - this is the Supreme Way.
He also firmly rejected all dualism, especially between the “ordinary” and “enlightened” states:
If you would only rid yourselves of the concepts of ordinary and Enlightened, you would find that there is no other Buddha than the Buddha in your own Mind. The arising and the elimination of illusion are both illusory. Illusion is not something rooted in Reality; it exists because of your dualistic thinking. If you will only cease to indulge in opposed concepts such as ‘ordinary’ and ‘Enlightened’, illusion will cease of itself.
Since all is Buddha-mind, all actions reflect the Buddha, are actions of a Buddha. Huángbò's teaching on this reflected the Indian concept of the tathāgatagarbha, the idea that within all beings is the nature of the Buddha. Therefore, Huángbò taught that seeking the Buddha was futile as the Buddha resided within:
If you know positively that all sentient beings are already one with Bodhi [enlightenment, Supreme Wisdom], you will cease thinking of Bodhi as something to be attained” 
Huángbò was adamant that any form of “seeking” was not only useless, but obstructed clarity:
Sentient beings are attached to forms and so seek externally for Buddhahood. By their very seeking they lose it.
'Studying the Way’ is just a figure of speech [...] In fact, the Way is not something which can be studied. You must not allow this name [the Way] to lead you into forming a mental concept of a road.
According to the accounts, Huángbò avoided clinging on written texts. This is exemplified by the following story:
What Huángbò knew was that students of Chan often became attached to “seeking” enlightenment and he constantly warned against this (and all attachment) as an obstruction to enlightenment:
If you students of the Way wish to become Buddhas, you need study no doctrines whatever, but learn only how to avoid seeking for and attaching yourselves to anything.
But although Huángbò often told students against dependence on textual practices, pointing to the necessity of direct experience over sutra study, his record shows that he was familiar with a wide selection of Buddhist doctrines and texts, including the Diamond Sutra, the Vimalakīrti Sutra and the Lotus Sutra.
Huángbò was also noted for the manner of his teaching, incorporating the hitting and shouting pioneered by Mazu. There are a number of instances in the record of Huángbò slapping students.
The Blue Cliff Record tells the story of the future emperor of China, hiding in the Chan community as a novice monk, receiving slaps from Huángbò for questioning why Huángbò was bowing to an image of the Buddha.
The most famous instance was when Linji Yixuan was directed by the head monk, Muzhou Daoming, to question Huángbò on the meaning of Buddhism after Linji had been practicing in Huángbò's monastery for three years without an interview. Three times Linji went to Huángbò and three times the only answer he got was a slap.
You people are just like drunkards. I don’t know how you manage to keep on your feet in such a sodden condition. Why everyone will die laughing at you. It all seems so easy, so why do we have to live to see a day like this? Can’t you understand that in the whole Empire of the T’ang there are no "teachers of Zen"?
A monk stepped forth and asked, "How can you say that? At this very moment, as all can see, we are sitting face to face with one who has appeared in the world to be a teacher of monks and a leader of men!"
Please note that I did not say there is no Zen. I merely pointed out that there are no teachers!
While Huángbò was an uncompromising and somewhat fearsome Chan teacher, he understood the nature of fear in students when they heard the doctrine of emptiness and the Void:
Those who hasten towards it [the Void] dare not enter, fearing to hurtle down through the void with nothing to cling to or to stay their fall. So they look to the brink and retreat.