The Ember of the Abyss is the missing variable in every equation the celestial system had built around mortal warlocks. Most mortals, by the long arithmetic of creation, simply cannot touch celestial power directly. Their flesh is not made for it. Their souls are not made for it. The few who have tried have ended badly, and the celestial scribes have, in the older centuries, not even bothered to write their names down. The Ember broke that arithmetic.
It is small. It is faintly luminous. It would, in a stranger's hand, look like an unremarkable green-glowing fragment. Drexel located it by interrogating the dead — the dead, he had learned, remembered things the living had been kind enough to forget — and he dug down beneath the stones of a long-dead kingdom on Kolonoth and brought it up. He understood its value the moment he held it.
It does three things. It stores essence. It amplifies magic. It permits a mortal to touch celestial power directly — to take it, hold it, bend it, redirect it. The third property is the one no celestial doctrine had a counter for. The instant Drexel activated the Ember, Heaven felt him. Grandex sent three angels to retrieve it. Drexel did not kill them. He stripped their essence and bound it into the Ember through his staff, and the Ember has held the bound, almost-silent signal of those three angels ever since.
It also, it has since been demonstrated, opens portals. That is how Drexel reached Celesterra.