Soulburn is a Casterwill spell which lets allows the caster to burn up an amount of life force in order to regain energy for spells. It is dangerous, as the spell can cost the caster's life, so Soulburn is rarely used.
History[]
Sophie Casterwill used Soulburn just after her Sabriel's Amulet was destroyed while fighting the Professor while trying to prevent him from summoning Overlos, the Legendary Titan of Immortality. S1E26
Nimue Casterwill later used this spell during the Blood Spiral attack on the Fortress of Iron Will in order to cast the powerful Sacroblade spell, even when doing so cost her her life. In light of Nimue's sacrifice, Sophie then used the spell in order to defeat Kiel with Powerbonded Sorcerel and to drive back the Spirals. S2E40
While exploring Tír na nÓg, Sophie was about to use the spell as a last resort against the Firbolg and Dullahan Titans so that she would be able to invoke Phoenix. She was stopped at the last moment by Lok Lambert, who had returned from the Inter-dimensional World, and Dante Vale. S2E48