Strength and other abilities
Graendal is surely among the strongest female channelers ever. But we have some discrepancies if her true level of power is 3(+10) or 2(+11).
In 'The Wheel of Time Companion' the entry about Graendal classifies her strength in the power at level 3(+10), putting three female forsaken above her (Lanfear/Cyndane, Semirhage, and Mesaana) as well as some channelers for the light (Alivia, Sharina Melloy, and Talaan din Gelyn), and on an equivalent level with El'Nynaeve ti al'Meara Mandragoran.
But again in TWoTC when reading the entry about Talaan we find a contradiction to the entry cited above, in fact we can read that the potential power of Talaan is 2(+11) "higher than Nynaeve's or Moghedien's and equal to that of Graendal, Messana and Sharina."
The second entry is more in line to what is described in the main novels of WoT were Graendal is always described as being the second strongest among the female Forsaken (even if it could simply mean that her combative abilities more than compensate for her lower level of raw power).
Graendal in the Age of Legends was famous as a great psychologist and a noted ascetic. After her conversion to the Shadow, she became a master manipulator and an extraordinarily skilled user of Compulsion weaves.
She is beautifully full-figured with elaborately curled red-gold/sun-colored hair. However, she is described as merely "plumply pretty" beside Lanfear.
Nindar was a famous psychologist for nearly four hundred years, and was known to heal mental afflictions even the use of the One Power could not cure. She was also a distinct ascetic, living an unadorned life. So fervent were Nindar's beliefs that she thought that all who could not live up to her
standards were morally inferior. She thought that the life that most lived was simply too complicated and fast paced. While Nindar was admired by the general public for her ascetic lifestyle, those who knew her well had no love for her since, after all, they could not live up to her standards either.
There is no evidence that Nindar's change of character was influenced by the Shadow, although she was the second of the Forsaken to swear her vows to the Dark One. Rather, it seemed that she realized that the world could and would never live up to her moral standards.[verify] Thus, she became the exact opposite of those beliefs, wearing the most fashionable garments and taking the most perverse sensual pleasures. She remembers her first steps towards The Shadow being full of pain.
Her alter ego, Graendal, is a creature of decadence and excess. One of the most physically beautiful women to have ever walked the earth, she habitually wears revealing and seductive clothing and expensive jewelry and keeps only the most attractive men and women near her as her servants. She is very particular about her servants, often choosing people of great status, power, or renown and reducing their minds to empty husks (though her standards of beauty still apply).
After she escaped the Bore, she hid herself in Arad Doman under the disguise of the ailing Lady Basene. She teamed up with Lanfear, Sammael, and Rahvin to try to turn Rand al'Thor to the Shadow. Shortly after Rahvin's death, she secretely murdered Asmodean in Caemlyn Palace, perhaps in a panic at her presence being discovered. Her identity as Asmodean's murderer is left unknown for a long time. It is finally revealed by Shaidar Haran when he blames Graendal for the destruction of three of the Chosen. While he mentioned only Mesaana, it is clear she is also responsible for the deaths of Aran'gar and Asmodean.
She continued her alliance with Sammael, and showed him the Sh'boan and Sh'botay she had captured from Shara hoping to fool him with a red herring. The intention of the subterfuge appears to be to aim, and possibly launch, Sammael directly at Rand, not to hide the activities of the other Chosen. She hoped that Sammael would either kill Rand, against the directive of the Great Lord, or die in the attempt, ridding herself of a burdensome associate. Sammael later tricked her into revealing where Mesaana was residing and said that he had made a truce with Rand, claiming that he would sit back and watch the other chosen destroy each other. She accompanied Sammael when he meets with the Shaido Wise Ones and gave them a callbox. He referred to Graendal in front of them as Maisia, a name that seemed to infuriate her. She then helped Sammael to disperse the Shaido all over the continent to help spread chaos.
In Arad Doman
Graendal took advantage of the kidnapping of King Alsalam Saeed Almadar by Elaida's Aes Sedai, and sent out confusing and misleading battle orders to Rodel Ituralde in the king's name. The orders were to do exactly the opposite of what might have ended the war with the Dragonsworn in Arad Doman, thus fulfilling the Dark One's command to "let the Lord of Chaos rule."
After Sammael's death, Cyndane and Moghedien were sent to Graendal as messengers. They reported that she was no longer a free agent and told her that she was now under the command of Moridin, a supposedly unknown man who had been named Nae'blis. Infuriated and incredulous that a man she did not even know had apparently become Nae'blis, she placed both of them under Compulsion. She then ordered them to tell her the truth, as she believed that Moghedien was simply attempting to manipulate her with false tales of the supposed Nae'blis. Shaidar Haran arrived moments later to confirm Moghedien's story, and was amused and approving of Graendal's immediate subservience.
She attended the meeting with the other Chosen in Tel'aran'rhiod, which was made to look like the Ansaline Gardens. There she was told that Rand is not to be harmed and that Matrim Cauthon and Perrin Aybara are to be killed if found. Aran'gar flirts with her and proposes an alliance with her.
As reward for working so well for the Great Lord she is summoned to Moridin's meeting with Demandred and Mesaana in the Blight. There, Graendal is allowed to hear everything. After the meeting, Moridin hints to her that there will be a place, a promotion for her, subordinate to him but above the rest of the Chosen if she causes Rand great anguish, while not harming him. She must not allow Rand to restore order in Arad Doman. 
Delana Mosalaine and Aran'gar are residing within Natrin's Barrow with Graendal. After Aran'gar's failure in losing control of Egwene al'Vere, she is placed to serve Graendal. Graendal reveals to Aran'gar that the Dark One has begun to favor her and creates a weave of Air formed from the True Power. However, she keeps quiet the fact that it is only a sliver that she can work with. When Ramshalan arrives, Graendal orders Delana and Aran'gar to place Compulsion on him and send him back to Rand. She watches Ramshalan make his way back to Rand and Nynaeve through a special weave that allows her to look through the eyes of a dove. When Nynaeve checks Ramshalan for Compulsion, Graendal then realizes that Ramshalan was bait. She creates a gateway and leaps through, shielding Delana and Aran'gar and leaving them to die. She escapes before al'Thor uses balefire on her fortress out of existence. Ramshalan is now freed of the Compulsion immediately after the balefire struck the fortress. Thinking that Graendal was the one to put Ramshalan under Compulsion, Rand takes this change to be confirmation of her death by balefire. In reality, though, it just indicates that Delana and/or Aran'gar died. It is also revealed that Graendal was the killer of Asmodean.
Graendal portrait by Seamas Gallagher
Graendal retreats into hiding on a desolate island in the Aryth Ocean. Moridin finds her and chastises her for her failure and for causing the death of another Chosen. She attempts to redeem herself by promising the death of Perrin Aybara, which Moridin agrees to allow her to attempt, loaning her a dreamspike and the services of Slayer. However, when Slayer fails to kill Perrin, the blame falls upon Graendal, as Slayer was merely the tool being used by her. Shaidar Haran arrives to punish her for her failure.
Graendal is killed and resurrected for her failure. When she is next seen, she is hideously disfigured and astonishingly ugly; Moridin introduces her as Hessalam, without forgiveness in the Old Tongue, and says her old name is not to be spoken of again.
The name Graendal may be an allusion to Grendel, one of the three antagonists in the epic poem Beowulf, along with Grendel's mother and the dragon. However the Grendel from Beowulf is a male monster/fighter and bears no resemblance to the Graendal of WoT; rather it is Grendel's mother who's been repeatedly portrayed in film as a shape-shifting seductress more fitting with Graendal's description. However, Graendal's final appearance does have monstrous characteristics, and her monstrosity has already existed on the inside since the day she joined the Shadow.