Taldeer was a Farseer of Craftworld Ulthwé and the leader of the Eldar strike force sent to Lorn V in anticipation of an impending Necron attack. She and her strike force saved General Sturnn's men on two different occasions, without them knowing until much later.
Later, Taldeer led an Eldar strike force to destroy the Necrons that had arisen on the Civilised World of Kronus, that the forces of the T'au Fire Caste were trying and failing to keep at bay. Farseer Taldeer's presence on Kronus attracted the attention of the Imperial General Lukas Alexander, who was tasked with chasing down Taldeer after her manipulation of the Imperial Guard on Lorn V.
With her Spirit Stone recovered, she was placed within the shell of a Wraithknight piloted by her twin brother, that would participate in the Eldar campaign on the world of Acheron alongside Farseer Macha.
Not much is known of Taldeer, other than that she was first seen in action on Lorn V, where she helped the 412th Cadian Shock Troops Regiment of the Imperial Guard gain a foothold against the forces of Chaos and later the Orks. She also led her forces successfully through the Lorn V and Kronus Campaigns.
In the Dawn of War series of novels, Taldeer is portrayed as a member of Biel-Tan's Seer Council and, as is common with Eldar Farseers, a female of breathtaking beauty and elegance. She forms an alliance with Blood Ravens Captain Gabriel Angelos in order to prevent the destruction of a Webway Portal on Lorn V and defeat the reactivation ("Ascension") of a C'tan and its Necron army.
She is seemingly killed in action during that campaign, though the Harlequin Solitaire in that story claims to have saved her. If she survived, she was horribly scarred and blinded from injuries sustained from exposure to the Warp and from a crash landing and subsequent explosion.
In Dawn of War II: Retribution, Taldeer comes in the form of a Soulstone. It is revealed in the Eldar campaign that she was taken prisoner by the Blood Ravens, most likely following the events of Dawn of War: Dark Crusade where she was "interrogated" by the Blood Ravens under the supervision of Azariah Kyras. According to Kyras, she was tortured and finally executed on Kyras' order.
The exact details as to why her Soulstone was on her person at the time of her execution are unknown, but it is later revealed Kyras kept it as a trophy on his personage which the Eldar recover during their campaign. She is the twin sister of Ronahn, an Eldar Pathfinder who aided Farseer Idranel and later Autarch Kayleth during the Aurelian Crusades.
It was Ronahn who recovered her Soulstone and tried to bring her back to their craftworld. Unfortunately, she was apprehended by the Autarch Kyre after learning of a prophecy regarding the Spear of Khaine.
After being freed by Macha but with their warhost isolated by Kyre, she ordered Ronahn to plant her inside a Wraithknight that Ronanhn would pilot into battle on the world of Acheron during the Acheron Campaign.
We have taken the game storylines to be the more canonical of the two.