In my someday-to-be-published book, I divide the figures seen in temples into five categories: Buddhas, Bodhisattvas, Kings and Generals, Arhats, and Historical Personages (founders, monks, etc.).
Here's a little surprise: In a typical Chinese temple, fully six of the first seven figures encountered fall into the relatively-minor third category: Kings and Generals. The Mountain Gate often has the two generals, Heng and Ha; and while the Buddha-of-the-future Maitreya ("The Laughing Buddha") occupies the central space in the next hall, he is almost entirely surrounded by five figures: one general and four kings. (In fact, it's usually called the Tian Wang Dian, "Heavenly Kings' Hall"--or some variation perhaps including "Four" or "Great"--despite the obvious importance of Maitreya.)
So let's take some time to meet these Si Da Tianwang, "Four Great Heavenly Kings."
First, some terminology: the Chinese term Tianwang is a word-for-word translation (a calque) of the Sanskrit word Devaraja, which is literally "God King." "Heavenly King" is the more popular rendition into English.
As we will discover, the Kings have multiple layers of significance. Though we encounter them as temple guardians--sort of backing up Heng and Ha in the Mountain Gate--they are so much more. In the following sections, I've broken their traits out into bulleted lists. Here are the categories:
Position: The Kings are placed in four corners around their hall. In most (but not all) temples, you will find the same King in the same position: left or right, front or back. I will give you their most common placement.
Meaning of Sanskrit name: In Indian tradition the Kings served as the god Indra's subordinates, one on each side of Mount Meru, where they protected the world from attacks by the malicious Ashuras. The Central Asian monk and translator Amoghavajra (705-774) introduced them to China as temple guardians (based, they say, on a dream he had). Each has his own special power.
Pali version of name: This is just the Pali version of the Sanskrit name.
Associated Hindu god: As you'll see in a poem below, there are four associated gods, and each is connected to one of the Heavenly Kings.
Planet: Each Hindu god was associated with a planet (as are ours).
Chinese name and meaning: Here I give the two-character name, its literal translation, and an expanded meaning.
Chinese folk name: In addition to their Buddhist veneration, the Four Heavenly Kings have also been adopted into folk tradition. Each name starts with Moli, meaning "supernatural power," and adds an attribute.
Alternate name: There are also sobriquets for some of the Kings.
Japanese name(s): Often simply the Japanese pronunciation of the Chinese characters for each King's name, these may be more familiar to some westerners.
Direction: Each King presides over one of the cardinal directions.
Color: Each King has associated colors, sometimes seen on the faces of their statues. These may vary in different traditions, so I have given the most common ones.
Holding: Each King is seen holding one or more objects, again with some variations; these are in some way related to that King's power(s).
Negative aspect: Each of the implements held by the Kings has both positive and negative associations; after the positive, I give the negative.
Palace material: Each King lives in a palace made of different stuff.
Continent ruled over: Each King rules over one of the four (imaginary, except for one) continents in Indian tradition.
Supernatural subjects: In addition to the (extraordinary) human beings who occupy the four continents, each King also rules a group of supernatural beings.
I must emphasize that some of these associations--direction, color, what the King is holding, etc.--can be different in the heterogeneous traditions of Chinese Buddhism. What I have offered here is a sort of average, but your miles (as they say) may vary.
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Here we go! We'll take them in the order in which they're found in the hall, starting at the front left and moving clockwise. All images were taken at Longxing Temple, Zhengding, Shijiazhuang, Hebei.