Vengeful Spirit

 

How to Acquire
It is given by the Snail Shaman, located in the Forgotten Crossroads, after defeating the False Knight.

Modifiers
TBA

Upgrade
It can be upgraded into Shade Soul at the Soul Sanctum. See more on Shade Soul.

Game Progress
With the Vengeful Spirit spell, the player can kill the Elder Baldur blocking the way to Greenpath.

Trivia
Interestingly, the game files refer to this spell as "Fireball":

"A desire for revenge is seared into the SOUL of those who fell in battle or were wronged in life. A Shaman can concentrate this desire and cast it out into the world to burn away whatever stands before them."