Aghdashloo is best-known for her role on all six seasons of The Expanse, playing Chrisjen Avasarala. Her performance on that show was highly acclaimed, and led to her being cast in numerous geek-friendly projects. However, her acting career goes right back to the late 1970s, when she debuted in films in her native Iran. She moved to the UK and then the United States after the 1979 Revolution and began acting in projects there, achieving a breakthrough for her role in House of Sand and Fog (2003). Her TV roles included ER, Grey's Anatomy, 24, House of Saddam, The Punisher and Grimm. In video games, she also voiced Admiral Shala'Raan of the Quarian Fleet in Mass Effect 2 and 3, Lakhshmi-2 in Destiny and Destiny 2, and Roshan in the Assassin's Creed franchise. She reprised her role as Avasarala in this year's The Expanse: A Telltale Series.
Aghdashloo was fan-cast by Wheel of Time fans for the role of Cadsuane, a powerful Aes Sedai who cannot stand the strictures of the Tower so prefers to operate independently of its hierarchy, something that has gotten her in trouble with the sisterhood. In the books, Cadsuane is initially a difficult-to-read figure who may be an ally but also an enemy of Rand al'Thor. Aghdashloo subsequently responded positively to fans on Twitter suggesting she audition for the role, leading to an exchange with showrunner Rafe Judkins. She was subsequently spotted on location when filming for Season 3 began in Prague.
According to WoT Up!, Aghdashloo is actually playing the role of Elaida, a different Aes Sedai. Elaida is the Aes Sedai advisor to Queen Morgase of Andor, thanks to her gift of the Foretelling, comes to believe that the royal house of Andor is pivotal in events to come. She relocates to Tar Valon, where Morgase's daughter Elayne is studying to be Aes Sedai, to better guide her education.
Although I think Aghdashloo would be a better Cadsuane than Elaida, she would still be excellent in that role. It may be that Cadsuane's role in the show will be reduced compared to the books, in which case casting a lower-profile actress for that role might make more sense (Cadsuane has been mentioned several times in the series, so will probably appear in some form), whilst Elaida will be a more important figure who debuts earlier and plays a bigger role in events.
We await full confirmation from Amazon as to Aghdashloo's casting and what role she will be playing, but overall this is excellent casting news.