Strength and abilities
In the 'WoT Companion' her strength in the One Power is classified 1(+12), making her stronger than all other female channelers except for Alivia and Lanfear (prior to her stay with the Finn), both of whom match her in strength. Though Graendal has been described as the second most powerful of the female Forsaken, this apparent contradiction could simply be the result of Graendal having significantly greater combative skill than Semirhage despite being weaker than her in the Power.
Semirhage was among the most prominent Restorers, or healers, in the Age of Legends, and even before turning to the Shadow she was known to be cruel, often needlessly causing people extra pain when Healing them. Among the Forsaken she was the most depraved and sadistic, known for her unsurpassed skill with torturing and the pleasure she took in it, and she was in general the one people were most afraid to fall into the hands of for that reason. According to Graendal, Semirhage was utterly corrupt even before her conversion to the Shadow. She also appeared to have been something of a masochist, as she thought to herself at one point that she had been looking forward to being tortured by Rand al'Thor's Aes Sedai after her capture.
Asmodean thought that she might know how to block Gateways from closing. She was rather poor at working with the weather. She could play many games such as sha'rah, but was indifferent to those that did not involve actual people.
She enjoyed doing needlework, especially beautiful patterns such as flowers.
She was slender and taller than most men (6'1 1/2), yet proportioned such that her height was not striking when she did not stand next to other people. She had a beautiful charcoal-black face with large black eyes and full lips. She had short wavy hair. She was believed by the other Forsaken to have always dressed in black because Lanfear always dressed in white, though none of them were ever willing to say it directly to her. Cadsuane Melaidhrin described her as having a "willowy" build.
Semirhage was the most gifted healer in the entire Age of Legends, an occupation then known as a Restorer. She was whisked all across the world because she could heal anything short of death. Revered as she was, Semirhage sometimes gave people a little extra pain after healing, but few said anything about it due to having been saved from death by her. Some people Semirhage deemed undeserving of life, and those were not saved but killed instead, doubtlessly in an unpleasant fashion.
When the Hall of Servants found out about her sadistic crimes they offered her a choice: be bound as a criminal by a Binder not to take her 'pleasures' or to be severed. Semirhage was furious at the Hall for thinking more of "common people" over someone as important as her. She chose a third option. She was among the first to pledge her soul to the Shadow.
Despite not being particularly powerful when compared to many of the other Forsaken, Semirhage's talents made her one of the most feared. One of the first of Lews Therin Telamon's memories to surface in Rand al'Thor was the memory of Semirhage being very poor at working with weather.
War of Power
During the War of Power, territories governed by Semirhage were often marked by terrible cruelty - even compared to territories governed by other Forsaken. She forced people in cities to help torture each other. She used this in her studies about breaking people using pain, and experimented on ways to kill, such as replacing people's blood with various foreign substances.
Semirhage used this sinister knowledge on those who had - in her eyes - made her turn to the Shadow, and was filled with enormous satisfaction for each one of those from Hall of Servants who had thought they could judge her, as each swore their loyalty to the Dark One. She became widely known as the Lady of Pain. So horrific and widespread was her reputation that people were known to bite through their own wrists rather than face her. In fact, special precautions had to be taken to keep prisoners from committing suicide when learning they were to be handed over to Semirhage.
Semirhage discovered the technique where thirteen Dreadlords and thirteen Myrddraal could turn any channeler against their will to the Shadow. However, her preferred method of turning a person to serve the Shadow remained torturing the subjects into swearing allegiance to the Dark One. In addition to her pure enjoyment in inflicting excruciating agony, she thought that painful and perverse punishment was more effective - despite being more time-consuming.
Semirhage also headed a network within lands under the Shadow to root out traitors and spies who followed the Light, and she was extremely effective in that role. There was a time during the War that Semirhage was captured, but she frightened her guards so much that they smuggled her out of captivity.
Summoned together with the other twelve most powerful of the Forsaken, Semirhage found herself in the middle of the Hundred Companions' strike at Shayol Ghul, led by Lews Therin. Sealed inside the Dark One's prison, she slept for three thousand years.
Semirhage, Demandred, and Mesaana have likely been working together since before the War of Power. Semirhage had an intimate knowledge of the brain and its pain and pleasure centers, and used these to torture Cabriana Mecandes, an Aes Sedai of the Blue Ajah, into bending to her will, paving the way for Balthamel, recently revived as Halima Saranov. During the same incident, she killed Cabriana Mecandes's warder, over-stimulating the pleasure centers of his brain and not paying attention to her efforts due to extended thoughts about the other Chosen. She also receives direction from Shaidar Haran, feeling the same discomfort as all of the Chosen feel when encountering this strange Myrddraal, demanding all of the information she can gain from Cabriana Mecandes. After her release, it was Semirhage who sent Shadowspawn to counter Sammael's Shadowspawn in the Stone of Tear, where they were attempting to slay Rand al'Thor, the Dragon Reborn.
Semirhage posed as a soe'feia named Anath Dorje, replacing Neferi, who reportedly died falling down stairs. Appointed as Tuon's supposed Truthspeaker and retainer, Semirhage accompanied Tuon on the voyage aboard the Victory of Kidron in the Seanchan invasion force. They arrived at Ebou Dar. Semirhage's plan to manipulate Tuon was unsuccessful as the future Empress is strong minded and a "willful charge." After Tuon was spirited away by Mat Cauthon, out of Semirhage's control entirely, she went to Seanchan to carry out a new plan that played more to her strengths. She murdered Empress Radhanan and the entire Imperial Court, plunging Seanchan into chaos. She then paid a visit to Suroth Sabelle Meldarath, a Darkfriend, and told her to find and murder Tuon. Semirhage planned to set Suroth up as the new Empress of Seanchan - one who was completely under her thumb.
Meeting with the Dragon
Later, 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 was not to be harmed and that Mat and Perrin Aybara were to be killed if found.
Semirhage then masqueraded as the Daughter of the Nine Moons, Tuon herself, for a planned meeting with Rand. Her weaves of disguise were unraveled by one of Cadsuane Melaidhrin's ornaments in her Paralis-net. She was identified and captured, but not before blasting Rand with a fireball. The fireball amputated his hand, fulfilling one of Min's viewings. Afterwards Semirhage was captured by Rand and his companions, but he once again refused to execute a female Forsaken.
Semirhage pointed out that Rand heard voices, which was one of the signs of Rand's impending madness. She stated that it may be very soon that he would completely succumb to the madness and kill all those around him.
While imprisoned she was given to Cadsuane who tried to interrogate her, without success. Semirhage just ignored Cadsuane and others, including Merise Haindehl and Sarene Nemdahl, who tried to coerce information from the Forsaken without resorting to torture, which Rand had forbidden. Instead of answering questions, she talked about torture techniques and the violence she did to her prisoners, causing fear amongst her Aes Sedai jailers.
When Sorilea pointed out just how human she was in actuality, the Wise One got a brief reaction. It was Cadsuane who ultimately understood that Semirhage was too proficient in the arts of torture to succumb to it herself. Indeed, Semirhage seemed almost to anticipate with relish the challenge of torture. However, Cadsuane realized Semirhage's overblown ego was susceptible to humiliation. Consequently, Cadsuane systematically began to humble her and whittle away her larger-than-life reputation, beginning with a good old-fashioned spanking.
Soon afterward, Semirhage was released by Shaidar Haran. She was given the Domination Band by Elza Penfell, and together they used it to capture Rand. She caused great pain to Rand through the band before trying to make him choke the life out of Min Farshaw. In a panicked rage, Rand drew upon the True Power, broke the Domination Band, and then killed Semirhage and Elza with balefire.