“Calm yourself,” the shadow signed. “I have a plan,” he said. “There is a universally common weak point among all humans. I intend that we exploit it. Ruthlessly.”

A universal weak point? What could he possibly mean by that? It could be just about anything, but right now - Right now Kallen would take any weakness she could find!

[...]

Was that the plan? Because it didn’t make sense to her. Then again he hadn’t quite explained all of it. Just get him there. He’d said, implying that it would become apparent why when she arrived. But no, she couldn’t see it. What did he mean? What did he mean by universal weakness?

Another fact that was making her nervous was the invisible spirit apparently following this person around. It worried her for several reasons. What if he decided to send it out to attack while the shadow wasn’t around to warn her where the attack was coming from? All she knew was that the plan was to taunt him into following her to a specific location.

Regardless of what his plan was, she could see the target up ahead. A pier. Not many people around. People that they’d passed on the street had shot them funny looks, but what the hell this was a life or death struggle. She didn’t have time to worry about their curiosity at the moment.

She stopped on the pier, turned and stared at her reflection in the water. Here she was. What was the plan? It still didn’t quite fit in her mind. What did he have in mind. Universal weakness. What did that mean?

[...]

Inspiration struck Kallen like a bolt of lightning, and she must have hallucinated a shadowy hand pointing towards the water. No. It couldn’t be. Could that be the plan? It was insane! And yet, as she turned it over in her mind she saw it. This would work. It had to work. It couldn’t fail! She relaxed her body, took a single deep breath, closed her eyes.

[...]

Kallen dove forward like a tightly wound spring, colliding into that solid rock body with all her weight and strength. This move must have shocked Frederick, who knew that she had to be aware this move could not hope to hurt him. But it was not her intention to hurt him. A fact that he realised far, far too late. She had not hurt him. She had thrown him off balance.

Kallen thought she felt something in her shoulder pop, but she had other concerns for the time being: Pushing Frederick off the other side of the pier. An expression of utter shock filled his face and he sank beneath the waves like a proverbial stone. Where he fell, there was a tremendous splash, a large ripple on the surface of the water, several bubbles… And Kallen very quickly dove right after.

The human body is buoyant enough to float within water, only because we have air within our lungs. By lying flat on our backs we ensure no effort need be expended in floating. Just breath in and out, relax and stay barely afloat. But if the human body was heavier, were it made of stone it would sink into the murky depths without mercy, without hesitation. No matter how strong a swimmer you were. Archimedes Principle. A crueler calculation than it may first appear.

Sure enough Frederick surfaced and Kallen allowed herself a smile. He was out of his element. On land, he was untouchable. But in the water he was helpless. He had no choice but to drop his stone form lest he succumb to the “universal weakness”: The necessity of all animals on Earth, the ability to breath.

Deprive a human being of food for a few days, and they will begin to waste away. Deprive them of water and they will slowly wither until suffering from kidney failure. But deprive him of oxygen for a few minutes and anyone will keel over. Everyone knew this basic biological fact, and when submerged underwater the very first reaction anyone would have would involve doing whatever it took to reach the surface.

He was gulping in deep breaths of air, which made it easy to slip behind him. He was too busy coughing up water to notice her until the moment she covered that mouth with her hand and with the other was able to slip a knife into his back with about as much resistance as would be expected from flesh. He didn’t have time to react with anything more than a strangled whimper. No more stone form. Nothing but flesh versus steel. From this position it was a comparatively easy matter to slide the knife through the skin and muscle tissue of his back and into his jet black heart. Blood seeped out of the wound, and Kallen hoped against hope that she hadn’t been seen.

-Lulu's Bizarre Rebellion Chapter 6