Grim had the tattoo inked on his chest thousands of years ago. The tattoo was a depiction of his heart being locked in a cage. His heartbeat had always been normal but it was as if it had been shut down and the key to activate it was emotions. Since he had banned all types of emotions in his territories, there was no way to wake his heart up. It was pretty natural for him. He had been like this since he was created. He wasn't allowed to act as per his heart's desires. As his heart never desired, he never found it troublesome.