Yamantaka is the most wrathful highest yoga tantra yidam and one of the yidam practices of Lama Tsong Khapa. He is concerned with both illusory body and clear light meditation—the only tantra which does so. He has nine heads, symbolizing the nine categories of scripture, thirty-four arms making use of various implements, and sixteen legs, trampling the opponents of the dharma. He is a wrathful emanation of Manjushri, the Buddha of wisdom, and can either appear alone or with a consort, as seen here. If he has a consort, his consort is known as the "vampire lady" Vetali.
Yidam practices are among the most important in Buddhism, because yidams are personal deities which one meditates on at the point of death. If one can successfully visualize one's yidam at the point of death, the yidam will help you cross over the bardo into either enlightenment or an auspicious rebirth. Yamantaka is one of the most popular yidams.
The word "Yamantaka" literally means "slayer of death." The practice of Yamantaka is meant to overcome death. Not necessarily in the literal sense of a single body living forever like Count Dracula or Lord Voldemort, since the Buddha taught that death comes to us all, but in the more important spiritual sense of not being forced to take rebirth when and where you don't want to. Yamantaka also is meant to harness anger and hatred. When we are angry, it is as though we have no control over what we do or say. Or, we have to fight tooth and nail to make sure we don't explode. But when we practice wrathful deity practice, we can learn how to use anger wisely and with a paradoxical serenity.
Yamantaka is said to have been born when the people prayed to Manjushri to save them from death. A great and powerful yogi was meditating in a cave, when some robbers who had stolen a yak entered the cave and began to eat it. They suddenly noticed they were not alone, and they decided to kill the yogi to make sure he didn't bear witness against them. The yogi, who was on the verge of attaining enlightenment when he was killed, became overcome with intense anger, and using his magical powers, he attached the yak head to his neck and killed the robbers. Still angry, he roamed around the countryside, killing everything he came across. The people, terrified, prayed to Manjushri, who took the form of Yamantaka and destroyed the yak-headed monster.
There are a number of things associated with the story of this "yogi" in the cave regarding why and how he got it wrong. I'm of the particular opinion that the yogi got it wrong b/c he was really, in his heart, still a shy, little boy who was programmed never to raise his voice. If he would've just spoken up, and said, "HEY! You mother-f...ers, get the F out of my CAVE!" the robbers probably would've been so startled, they'dve left.
It's also a matter of debate what, exactly, happened to the yogi once the Vajra-Terrifier gored him in his stomach. Did he then get enlightened and instantaneously become the Vajra-Terrifier? That is certainly one possibility. (It probably would work with some enemies of the Dharma attempting to be yogis.) I am of the opinion, however, that there was an incalculable aeon of bodhisattvahood missing in this equation. There are certainly some kinds of teary-eyed, startled, and saucer-eyed Buddha-children in this world who would inevitably ask, "Why?" They may require special attention from Vajra-terrifiers in perhaps a more peaceful form.
Marilyn Monroe might be of some help in this equation. Consider some of the things she went through, and some of the people she inspired. Consider, alternatively, the quote from the Fifties: "To the moon!" Well, there are a lot of women in this world who might actually want to visit the moon. Maybe. Also, there may be some men who, as little children, become very interested in visiting all the places of the universe which could support human life - places such as the moon, mars, Titan (moon of Jupiter), and so forth. But they need special training, particularly involving their original motivations. An astronaut could literally live or die, at extremely often and regular points in time while in space, simply as a result of his or her original motivations.
This, perhaps, gives some clue as to why a Vajra-terrifier might want to pay particular attention to what he/she is doing with regards to killing "enemies." Some "enemies of the Dharma" who are attempting sincerely to achieve enlightenment can be reminded of their commitments to the lives of their students, and the accuracy and compassion of their dharmas, through one swift blow from the Vajra-Terrifier. Others, on the other hand, may require a very long tour of all the places they have and will affect with regards to their peculiar dharmas/Dharmas until they become thoroughly convinced of their original intent to become enlightened, as well as all its implications in the past, present, (particularly the present), and, of course, the future.
From personal experience, Vesantra's Meeting the Buddhas, and Tsem Tulku Rinpoche's "Advise to a Yamantaka Initiate."