The True Power is a source of energy similar to the One Power but which finds it's source in the Dark One rather than the Creator. It is not differentiated into male and female halves, which work constantly with and against each other, but rather as one reservoir of power that is available to members of either sex. The True Power is also only available to Channelers whom the Dark One allows to wield it.
The True Power was first discovered (at least in recorded history) as a "unified power source" that researchers at the Sharom had identified as a possible alternative to the One Power. As the One Power's division into male and female halves was believed to have been inhibiting further scientific advancement, it was hoped that this problem could be circumvented by accessing and wielding this new source of power. An experiment performed by Mierin and Beidomon managed to breach the barrier containing this energy source, but the experiment caused a violent explosion, destroying the Sharom and creating the Bore. It was discovered later that the source of this new power was the Dark One, which led to the War of Power.
Since only those sworn to the Shadow can tap into the True Power, very little is known for certain about its attributes. It can be used much like the powers of saidar and saidin, although both genders can access it equally well. The True Power can perform many feats that the One Power can, although it does them in such a way as to betray the energy's chaotic origins: a gateway created with the True Power causes a subsonic "scream" as the Pattern is ripped open; the Healing performed on Lews Therin Telamon just before he died was much more painful than ordinary Healing, and was said to "darken the sun" and to "sear his body" when it was forced on him by Ishamael. However, it was effective in curing Lews Therin of his madness. Use of the True Power is completely undetectable to any channelers, even to those who could wield the True Power, unlike saidar and saidin. However, the gholam, a being designed specifically to hunt channelers, could feel it in a somewhat similar manner to the way in which it could feel the One Power, but it could not recall the meaning of said feeling.
It would appear that the True Power is more inherently destructive than the One Power. This is evidenced by the fact that it seems to have the ability to destroy objects that are impossible to harm with the One Power; a Domination Band made of Cuendillar that would not even heat up when exposed to intense weaves of Fire can be destroyed by a simple spear of Fire and Air if one wields the True Power. Additionally, use of the True Power by the Dark One himself is very probably responsible for the degradation over time of the seals on his prison. Graendal once gave a general evaluation of its destructiveness with the statement "whatever the Creator could create, the Dark One could destroy." It is described as being a hundred times as violent as saidin; even holding it is tortuously painful. In The Dragon Reborn, Ishamael demonstrated the ability to float above the ground using the True Power, another feat that is said to be impossible with the One Power. Graendal also stated at one point that there are some weaves that can only be created with the True Power, which was later confirmed by Demandred when he thought, while using the True Power to see through the eyes of a bird, that such a feat could not be performed with the One Power.
It seems not to have the same limits as the One Power as to how much of it can be drawn. Rather, the Dark One himself appears to control how much a channeler can access. Rand, who was able to draw upon the True Power due to his strange link with Moridin, described the amount of energy that he held when first grasping it as being similar to drawing Power from both Choedan Kal, likely reflecting the level of access to the True Power that Moridin himself had. Before he was reborn as Moridin, Ishamael presumably drew enough of it to overpower Rand wielding Callandor before he was run through with it. Conversly, Graendal cannot draw upon it very heavily, as she is not as favored by the Dark One as Ishamael/Moridin. Even Demandred, who was very probably the second most favored of the Forsaken, was only able to draw upon a small amount of it.
It also appears to generate a physical warping of the Pattern surrounding an individual; Rand was described as having a dark aura that could be almost more sensed than see. This aura may have even caused an unusual decay of objects around him when he was feeling strong emotions (fresh tea becoming immediately spoiled for instance), and it produced feelings of unease in those around him when it manifested. With Rand, this aura could only be seen in one's peripheral vision, and if one attempted to gaze at it directly, it vanished. A much more evident manifestation of this aura was Ishamael himself. During the time when he called himself Ba'alzamon, he was almost constantly surrounded by an intensely dark aura which grew darker the stronger his emotions became. As Moridin, the only evidence that he possessed the aura was seen when Aviendha saw him watching her group go through a Gateway and she felt great fear at his presence, though all he did was watch her from a distance. Also, in Towers of Midnight, Graendal had great difficulty controlling her fear in Moridin's presence, despite the fact that he was not doing anything threatening.
It is possible that extensive use of the True Power may give one the ability to communicate directly with the Dark One outside of Shayol Ghul. In one instance, Ishamael beseeched the Dark One for aid against a Callandor-wielding Rand, and the Dark One apparently allowed him greater access to the True Power in response. Additionally, Moridin appeared on several occasions to be able to receive instructions from the Dark One regardless of his location. No other Forsaken was ever seen communicating with him in this manner, and no other Forsaken used the True Power to anywhere near the extent that Moridin did (nor do they have anywhere near as much access to it). One of the wounds in Rand's side, the one given to him by Ishamael, is almost certainly a result of being burned with the True Power, and cannot be Healed by either the One Power or Rand's body's own natural healing processes.
Balefire does not appear to have any unusual characteristics when woven with the True Power versus the One Power except for the added ability to destroy cuendillar, nor does it require any special knowledge to weave, as Rand was able to form it immediately after accessing the True Power for the first time. Thus it is likely that there is no technical difference between creating a weave with the True Power versus the One Power.
Use of the True Power has a greater cost than that of the One Power. Unlike the urge to channel saidar or saidin, which can be controlled safely with willpower, it is believed that those who channel the True Power find themselves irresistibly drawn to consume more and more of it as time goes on (or as long as they are allowed to by the Dark One). Flecks and strands of blackness, called saa, appear in the eyes of a habitual user, increasing over time as the True Power is used. Eventually, the saa spread across one's entire eyes to color them completely black, at which point it is believed that the channeler would become totally and irreversibly insane. These flecks move across the eyes in greater quantities and with greater speed when one draws more deeply on the Power. For this reason, the Forsaken use the True Power extremely sparingly and in very small amounts, and they tend not to use it for tasks that can be performed with the One Power. The sole exception to this is Moridin, who wields the True Power to the apparent exclusion of the One Power and in phenomenal amounts. Why he is willing and/or allowed to do this appears to be a combination of his status as the Dark One's favorite servant, and the fact that he seems to be insane already. As such, the negative effects of the True Power on his mind are mostly negated.
When severe amounts of the True Power are used, in later stages, even greater deformities may occur, such as in the case of Ishamael before he became Moridin. It has been speculated that the appearances of Ishamael's mouth and eyes, which were replaced by yellow-green flame at times, may have been a result of overuse of the True Power. It is also very likely that the reason for Ishamael's body decaying as rapidly as it did after death was due to the effects of the True Power. When Ishamael was awakened from death and given the name Moridin, he did not stop using the True Power. In fact, he appeared to use it to the complete exclusion of saidin, and appeared to have been even greater access to it than his previous incarnation. As Moridin, he has entered the beginning stages of degeneration, with only saa appearing. It is however, worth noting that Moridin, despite having been given a fresh body with an undamaged/uncorrupted brain, was already far more insane upon his rebirth than he had ever been as Ishamael. Thus it is not known for certain how much of his madness can be blamed on overuse of the True Power versus the other factors that may have been responsible for it.