A newly discovered powerful Zero Numbering skill.
Soul Explosion is a self-sacrificial skill. It gives a large boost of power at the cost of detonating the soul of the person using the skill.
Kang Hansoo speculates that using it once fully would grant him mastery (thereby Transcendence in it) and also void his contract with the Demons of Demon's Gate, regaining his memories. But doing so meant instant death, hence there was no point.
A skill that gives immeasurable power to the person in exchange for the destruction of their soul. If the skill is used, the user's soul is destroyed in the process.