Side-effects
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.
Read more about this topic: True Power