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Unlike the One Power, which stems from the True Source, the True Power comes directly from the Dark One. Both men and women wield the same aspect of the True Power, unlike with the One Power's twin aspects, saidin and saidar, but any channeling of the True Power cannot be detected by anyone but the Dark One and those to whom he has granted access. Access to the One Power and the True Power are simultaneously dependent on and independent from each other. Only those with the ability to channel the One Power can channel the True Power, if allowed (though Shaidar Haran may be an exception, judging by some abilities he has been shown to have), but access to either source is independent from access to the other. For instance, when Semirhage linked with an unwilling Rand al'Thor in book twelve, The Gathering Storm, and controlled his use of saidin, he was still free to unconsciously tap into and channel the True Power.
While there is little evidence in the series of what it is like to use the True Power, what there is would suggest strongly that it is a destructive force, inimical to the Pattern. For example while weaving a Gateway a channeler with saidin cleanly cuts through the Pattern to bridge the gap. A channeler with saidar persuades the Pattern to reform so that the location the gateway is "from" is the same as the location the gateway is "to". In the only example in the books of a Gateway being weaved with the True Power, the Pattern "screams" as a hole is "torn" in it. The Dark One, Shai'tan, from whom the True Power stems in the first place, is the "devil" or "destroyer" figure in the Wheel of Time series. Also, the True Power can be much stronger than the One Power, as it can be used to do things that are impossible through channeling the One Power. For example, it was once used by Ishamael to cure Lews Therin Telamon's insanity, which was caused by the Dark One's taint on saidin; such an act was deemed impossible through the use of either saidar or saidin until an event in the Towers of Midnight where limited progress was made.
The True Power inflicts simultaneous pain and pleasure on the user. An external observer can see black flecks or saa pass through the user's eyes. These flecks become more common with use, and are therefore taken as a sign of the addiction progressing. The True Power is far more addictive that the One Power, and once the saa appear, death and madness from overuse is believed to follow soon. In the first three books in the series, Ishamael, the most corrupt of the Forsaken, is shown with fire raging through his eyes and mouth in many key scenes although this is intermittent with normal appearance. Ishamael may have also believed he was the Dark One. Whether this is due to the excessive use of True Power is unknown. However, if this is the case, it leads to interesting questions in the nature of the True Power itself on how much it could corrupt the mind.
Originally, all of the Forsaken could wield the True Power, but few ever did, due to its corrupting nature. Demandred himself clearly states that he only uses it with pressing need and very reluctantly even then. Only Moridin could wield the True Power after he had been reborn and named Nae'blis.