Will-o'-wisp

A Will-o'-wisp is an ethereal spherical being made of a glowing blue fire, which ignites souls to use as fuel.

Description
Will-o'wisps are ethereal beings of an unknown origin that produce blue and (rarely) black flames, which can ignite souls to use them as fuel. The remains of souls ignited by will-o'-wisps are known as "soul dust". Black flames burn 10x hotter than blue flames and spread with twice the speed.

The Seer
The seer's 4th eye form, Ignis, can generate will-o'-wisp flames.