In Game Description
Ember required for weapon ascension.
Chaos Flame Ember is an art of the lost city
of Izalith. Handled only by blacksmiths
knowledgeable in ancient methods.
Ascends +5 fire weapon to chaos weapon.
(chaos weapon can be reinforced to +5)
Chaos weapons are demon weapons augmented
by humanity which inflict fire damage.
Found in Demon Ruins, behind the Minor Taurus Demon mini-bosses, which appear after Ceaseless Discharge is defeated
Can be obtained with or without equipping the Orange Charred Ring. If opting for the latter, using Flash Sweat or having a high amount of health is recommended to survive lava damage - video
If going for a suicide run, using Hidden Body magic or the Ring of Fog will make it much easier by if the Minor Taurus Demons are present. Be aware that when chasing the player character, they may occasionally obscure the camera.