Transmutation

The method for releasing revenants was simple enough, this bird held on for so long, that it already reached its limits and started to decay. As soon as it ran out of magic to prolong its life any further, in another half an hour, it will be completely sucked dry of its life force.

Then, the revenant will appear before them.

"Why not just kill the bird?" Suddenly, a mage at the side asked.

Benjamin shook his head.

"Killing its prey during its feeding would only anger the revenant." He explained, "Although we made thorough preparations, who knows whether the spiritual energy barrier from the glass container could hold against the spirit? Besides, the spirit is fragile at the moment, what if when the prey is killed, it didn’t stockpile sufficient energy, plus the dark elemental environment wasn’t good enough, leading to its instantaneous demise?"

The spirits were unlike any other revenant lifeforms, it would be best for them to take some precautions in the matter.