Death.
If mana was like a violent river filled with all sorts of rapids and even a waterfall, then Ki would be a maelstrom...
The violent bone-crushing energy raced into Cain's body and tore him apart. Previously, he had been able to absorb Ki with little to no issues asides from enduring a ridiculous amount of pain. But now, the energy was going through every point of his body and filling it up.
Unfortunately for him,
His body wasn't strong enough...
To gain the ability to control Ki, one must possess a strong body that could actually bear the strain and damage that came with using this sort of violent energy. The reason why exercise is a good way of accumulating Ki is that it allows the body to develop a natural acceptance of Ki and more naturally incorporate it into its system.