Eostra was eventually killed by a spell cast by Narrander, a Soul Eater who had gone mad when his son died in the Great Fire, who summoned the Hidden People of the Mountain as she was trying to summon the Unquiet Dead, the souls of the dead Soul Eaters to serve her.
She was once a member of the Eagle Owl Clan and at some point became their Mage. It is unknown how long she spent with the Clan, though she left them after bringing a boy member of the Clan back to life (all of the Clan later died of illness leaving Eostra the sole member). Whilst in the Soul Eaters she kidnapped Narik in order to force his father, Narrander, to stay with the group. After the events of Souls' Night she went into hiding.
Thirteen summers after the Great Fire, Eostra returned with the remaining Soul Eaters and she was central in the Soul Eaters' plan to summon demons from the Other World and was presumably the one to gather them again. She was the bearer of one of the three fragments of the Fire Opal and was to be the one to control the demons into their bidding. She sent Seshru, Nef and Thiazzi to capture nine hunters from the forest and use them as the sacrifice to protect the Soul Eaters from the demons. Among them was Wolf, Torak's pack-brother. When Torak released the hunters and he, Wolf and Renn attacked the Soul Eaters, Eostra and Seshru opened the door to the Other World and the Eagle Owl Mage called the demons to her before Seshru was knocked out by Renn. With the demons under her control, Eostra was near to invincible, until Wolf knocked her over and made her drop the Fire Opal. Eostra sent her eagle owl with the three Soul Eaters to get the Fire Opal back. The plan failed when Nef sacrificed herself to destroy the Fire Opal, repaying her debt to Torak's dead father.
Eostra has no real personality. She seems souless, intent only on achieving immortality and power. In her youth, according to Krukoslik, she was obsessed with the dead and wanted to bring them back to use them to rule the living. She is cunning, apparently possessing a great intelligence which enabled her to manipulate Torak into her claws during Ghost Hunter. The only sliver of personality she shows is her interest in the dead, wanting to speak with and even control them. It could very well be that her dealings with the dead could have turned her into what she is now. As dealing with the dead lead to madness.
She has great control of Magecraft, being able to grip the entire Forest in her icy power. She has the knowledge to command Demons with the Fire Opal and can even bring the dead back to life. She would almost be invincible, if she were to become the spirit walker. Eostra was mentioned by Fin-Kedinn to Torak when he was explaining the soul eaters as the one who could raise the dead. A ability that makes her unique even among the other soul eaters is the power to snare and devour souls, this could be the reason the other soul eaters feared her. As they only took the name to install fear. As krukoslik explained, she pierces the victim's ears with a unearthly shriek, tearing the souls from the marrow and snaring them with her three pronged fork, then finally devouring them. It's unknown what happens to the devoured souls afterwards, though its likley they are destroyed or absorbed. This ability was perhaps her most feared power. It is not proven (although possible) she can teleport, as she seems to apear and disappear rather quickly, however this is never confirmed. She also appears to be able to see through her eagle owl, who seems to spy and gather information or in some way be able to spy on people. She was very possibly the most powerful mage to have lived and certainly the most feared.
As mentioned above, Eostra seems devoid of all emotion. She is both feared and hated by all of the Forest Clans and has no real connections to anyone. The only creature whom she shows any sort of bond is her Eagle Owl, though she is believed to just be controlling it anyway. Her sole purpose is to live forever and she seeks anything to help her further her aims, not shrinking from using Demons and Tokoroths.