Negative Energy destroys life force

That was because he was being infused with negative energy, which ate away at his life force. Zaryusu was trained to withstand pain, but he could not bear the terrible anguish that turned his veins to ice.

[...]

Where had he gotten that vitality from? The massive surge of negative energy pouring into him should have been steadily dissolving and consuming Zaryusu’s life force.

- Overlord; Vol 4; Chapter 3