Weakness.
Damien had always been thoroughly aware of his weaknesses, but they'd always been internalized.
The weaknesses that stemmed from his own lacking existence could be fixed through training and effort, so he never put too much stock into them.
Instead, he worked ceaselessly to rid himself of weakness and truly become untouchable.
However, there was one thing he'd forgotten.
As a practitioner, emotions were both a blessing and a curse. The presence of emotions allowed for the presence of determination and drive, and the presence of emotion allowed for expanded creativity which helped with comprehension.
However, at the same time, when these emotions were directed towards other people, they became a curse.
Strong hatred could be a source of determination, but it could also cause one to fall into the depths of despair and bottleneck their training.