The Tachikawa Sect is often made out as the villain in a lot of romance novels, but the novels are simply drawing on an extreme example and it does not represent the real Tachikawa Sect.
The fundamental methods of the Tachikawa Sect follow the main teachings of Shingon Buddhism and expand upon them. The methods of Tachikawa are not evil on their own by any means.
The representative “evil deed” that is taken up by others is the use of skulls as an object of worship, but really the use of skulls in rituals was not uncommon across Japan and in the Kamakura period the use of skulls was endorsed by many a Buddhist master, and the use of skulls was not considered to be something out of the ordinary.
Like this, the taboos of Shingon Tachikawa more or less align with those of fundamental Buddhist teachings.
The greatest taboo is to defy your master.
Buddhism is fundamentally a system where you [the disciple] attain enlightenment by using the methods left behind by the founder of Buddhism [the master] who became a Buddha, Shaka (Gautama Buddha.1 So the relationship between master and disciple is core in the Buddhist system. And Shaka preached that disciples must not doubt his ideas in any respect, and must only obey— the only time a person may have doubt, is before they formally become a disciple. It’s fine to doubt as much as you like before becoming a disciple, but it best to only become a disciple once all your doubts have been cleared.
So for the above stated reasons, Buddhism has always been centered on the relationship between master and disciple, and the power of Buddhism to reach enlightenment is fundamentally passed down through the “bloodline” (the unbroken and perpetual master-disciple link continuity2).
So essentially the only way to attain enlightenment through Buddhism is to become a very, very distantly connected disciple of Shaka, as it is he who has the power of Buddhism. So if you are excommunicated by your master, it means your link to Shaka will be severed, and all your ascetic practices (Buddhist training) until that point will be reverted to naught.
- ^ In Japan they usually say Shaka or Shaka-sama, but Gautama Buddha is his name in the Pali language (his original language, though now a dead language)) Seeing as the original Japanese text in this book writes it and annotates it as simply Shaka, I have left it as such throughout the whole book. The name Gautama Buddha would likely be familiar to more readers though, and is completely interchangeable with Shaka.
- ^ Not an actual parent-child style bloodline.