The Tome of Eternity was a sinister book written in the blood of a Black Dragon. It was created through the unholy alliance between Xostheron, Akaerna-Sagai and agents of the Black Dragonflight. Serving as a spellbook and as a magical manual, the Tome allowed its wielder to control Black Iron Golems and tap into the essence of shadow.
It was taken by the Chancellors to Alterac after the fall of Ythan'alai. There it lay hidden for generations, until Giren had Valentina von Mariendorf steal it and kidnap Minerva. Valentina took the Tome and Minerva to Gilneas, accompanied by Archerons. They were ambushed by assassins from the Azure Church of Gilneas. Minerva escaped with the Tome of Eternity, and was discovered by Tobijah Kruel.
Tobijah Kruel thus came into possession of the Tome of Eternity, which inspired him and fed his will to power, leading to the formation of the Church of Kruel. Minerva eventually tried to steal the Tome back from Tobijah Kruel, but was discovered. She led her loyalists towards Harrowdale, where her dreams had told her to go and take the Tome of Eternity.
Richard de Marmont, a Black Esarim, was sent after her with the Tome of Eternity. He was defeated and the Last Urubori used the Tome of Eternity to unchain the Black Iron Golems sent against Harrowdale.
The Tome was used by the Last Urubori to seal the Realm of Shadows.
A reading from the Tome of Eternity:
"... And so, the Black Dragon's fire pierced the Holy Light, rendering unto the Holy Light a gift. That gift we know to be the darkness within the Day - that which is called Shadow. And so, brothers and sisters, let the Light's most glorious incarnation, Shadow, awaken in your hearts."