The Endless Ember was a perpetually burning coal used as a stove on the Prince Alexander’s dining car. When the Red Zephyr knocked this car off the train, flipping it over, the ember set fire to most everything inside. The people inside were unable to escape, as the doors were jammed shut.
Lye Lychen and the Bell Metal Band later salvaged it from the crashed car. As it was dangerous, they had to contain it within a Shapeknight helmet; of course, Lyke soon put it right in his bag, next to the dormant god, also harmful.
A running joke on the show is that the Endless Ember is radioactive, as most real life rocks that constantly emit heat are.
In Episode 27, Lyke used the Endless Ember in his second desperate attempt at a ritual to prevent Virtue Mondegreen from taking the power of the Perpetual Oratorio into herself, and to trap the Oratorio in a pocket dimension. Unfortunately, it exploded, burning his hands horribly.