Actaea is a mysterious individual that appeared during the Horus Heresy.
Originally thought to be a blind disciple and oracle of the original Blessed Lady Cyrene Valantion, She was recruited by the Word Bearers to help find Fulgrim in preparation for Horus' drive on Terra. Actaea led them through the Webway and then to the Palace of Slaanesh, where Fulgrim was eventually found and bound by Zardu Layak using his True Name. All of these events were part of Lorgar's plan to usurp Horus as the anointed of Chaos, but in the end Actaea tipped off the Warmaster to the treachery. This combined with Layak freeing Fulgrim instead of using him to attack Horus as originally planned resulted in Lorgar's coup falling apart and the Primarch being banished. Afterwards Layak confronted Actaea and revealed that he not only knew it was her who informed the Warmaster, but that he had also looked into the history of of Valantion's Cult and no record of an individual named Actaea had ever existed. Layak asked just who or what exactly Actaea was, but she refused to answer and left.