Sylandri Veilwalker is a shadowseer of the Masque of the Veiled Path.
Official Canon Edit
Sylandri is a mysterious agent of the Laughing God, coming and going of her own accord. She appeared to the exiled Eldar prince Yriel, telling him of his future after he picked up the Spear of Twilight. She lured a Tyranid fleet away from a Webway portal by pretending to be an ambassador and luring a Tau force directly in their path.
During the 13th Black Crusade, she aided Belisarius Cawl at Cadia and helped the survivors to escape into the Webway. Therafter, she and her Harlequin forces continued to assist the Imperials by helping Roboute Guilliman reach Terra after he was revived, helping to fight off the forces of Chaos on Luna until reinforcements from Terra arrived.
TTS Canon Edit
Sylandri is working with Eldrad Ulthran and Inriam's Spectre to summon Ynnead and avert disaster for the galaxy. Should Eldrad run into trouble, she and Cypher were to go to Mars and find Cawl to push him into action.