First, let's explore the word tantra. Tan comes from Sanskrit and means condensing energy. There are 2 million and 1 mis-translations of the word, which literally stems from the word for loom or to weave. The best and most sensical translation is to condense and organize energy.
Tra means vehicle or movement.
So, while the literal meaning may be the movement of a loom or weaving vehicle, the true meaning is more appropriately understood as the focalisation of power/energy. This can be loosely understood in the way that a magnifying glass can focus relatively unfocused energy into something relativley potent. That is the meaning of "tantra." Nothing more or less special or majestic.
So, with that said, what is Tantra? Tantra is using the inherent tendency of the mind to become agitated to eliminate the mind. That is to say, tantra is taking advantage of the mind's inherent predisposition towards becoming agitated by certain things, then using that evoked energy towards a focused and directed purposed. Originally this purpose was to become englightened and realize truth, but due to the quite difficult nature of maintaining a pure purpose through this practice, it quickly became associated not with arousing energy for the sole use of uncovering hapipness/enlightenment, but arousing energy for other purposes- these have commonly been called "black magic," or supernatural powers known as "siddhis" in Sanskrit.
Tantra is nothing more than evoking or arousing energy through the mind's natural predipositions and then attempting to channel that energy for a specific use. Now you understand why it is like a loom gathering individual threads (energy) and putting them together to form something specific, right? Originally (as was said to be instructed by Shiva) this use was to realize Truth. Later, these practices were carried on in times and epochs in which the human mind was too predispositioned to falling into the trap of the very energies which were aroused by the Tantric rituals. For that, these rituals are often viewed as black magic. The most famous of which are known as Aghoras in India. In the original Tantric texts, given to Parvati by Shiva, it is said that nearly all of the time in our current epoch, the Ego is completely consumed by the ignorance which the energy/mental turbulence aroused generates. The most famous of which in Tantra is sexual energy. A person starts Tantra with the aim of using the aroused sexual energy for their own spiritual evolution, but later because of their own weakness this very energy consumes and destroys them in the form of desire for more sexual pleasure.
It is akin to a person begin to use heroin to relax. Even while it is true that a person can probably relax to a greater extent with heroin, in the end the practice will many times consume and destroy the practitioner because of the mental impressions left on a dense mind that later turn to extreme desire to repeat the pleasurable experience.
This is Tantra. According to Shiva, to begin a true Tantra practice is risky and only the very purest hearts coupled with the most determined and self-controlled minds attain success through a Tantric practice.