@strider92: Really? I thought Tempest was when he faced the Shadowseer and Grand Harlequin?
Nah in Tempest he had brief tussle with the Grand Harlequin and it got cut off for some reason that I don't recall. It was in Atlas Infernal he fought the Shadowseer.
The eldar warlock’s eldritch fashions were crafted from pure confusion and his every step quaked with disintegrated reality. The length of the Shadowseer’s leaf-shaped witchblade sang at Ahriman, forcing the Thousand Sons sorcerer back. Ahriman barely got his Black Staff in front of the devastating sword sweeps. Immaterial energies spilled from both weapons as the psykers clashed, forcing Ahriman further back. The archway bulkhead crashed back down at the Shadowseer’s mental insistence and the metal of the Impossible Fortress’s walls melted and dribbled down across the seals, fusing the door shut and trapping the sorcerer in the pancratitaph with his enemies.
With Ahriman’s power and attention directed very much on his unwelcome visitor, Czevak’s levitating body dropped to the deck. From his perspective on the floor the inquisitor watched witchblade and force staff smash, both physically and immaterially. Where they did, the fabric of reality tore. The disciplined grace and furiosity of the Shadowseer’s assault would have carved a bloodthirster of Khorne in two. Ahriman was more than pure wanton destruction, however. The Thousand Sons sorcerer was not only an ancient and devastating warrior, he was one of the most talented psykers in the galaxy. More important than either of these facts was the torrent of unbound ambition surging through the sorcerer’s veins. He was the impossible made incarnate.
Pg. 219 Atlas Infernal