Once upon a time, in a kingdom far, far away lived two princes. The two constantly fought and bickered, although one of them was always victorious, and always beat the other, even at simple games such as tag.
On one of the rare days when they hadn't started fighting over every little thing from the moment they had woken up, the younger had held out a small, silver key with the shape of a heart on the handle to the other, "I expect you to keep this safe for me, brother." With those words, he had turned around, walking away to do whatever it was that little princes did.
"That key you gave me all that time ago; I've lost it." Grinning, the elder brother tilted his head, awaiting the reaction of the younger- although it was far from what he had expected.
Tears fell down his cheeks, and the younger had picked up one of the kitchen knives (as that was where they currently were) before shoving his brother down, and thrusting the knife into his chest, screaming at the top of his lungs as he tore open the flesh, stabbing into it mercilessly for what seemed like hours, although it had been a much shorter amount of time than that.
He cut into his skin well after his brother had stopped struggling - had stopped moving for that matter - and only once the blood had pooled across the tiled floor and the shirt had been completely ripped up did he stop;
There it was, the key, kept around his brother's neck by a thin gold chain, which was now stained red with blood. Shortly after the tears had stopped, they started flowing again, and he merely stared for a moment, before collasping onto his brother's chest, sobs shaking his form. "I'm sorry, I'm sorry."
After he had destroyed the entire, worthless kingdom, the boy had gone out into the courtyard, dragging his older brother's body along with him until he had gotten to a suitable area, one that was surrounded by flowers, and far enough that the fire coming from the burning kingdom wouldn't reach them. After hours of digging with his bare fingers, he dragged the body into the four foot deep hole, and then placed a hand over the other's chest, fingers gripping onto the key. "This is the burden I'm leaving you, the one that you can never escape from."
A little more than twenty four years later, Belphegor stood in front of Rasiel, lips pressed into a scowl as he stared, stared for what felt like forever, yet was merely seconds. After he had shoved away his idiotic "kouhai", and Rasiel had shooed away his butler to "take care of" the rest of the Varia, the older of the two had reached into his shirt, pulling out the golden chain that held the key. "I've kept my end of the promise, Belphegor, now it's your turn."
Reaching into his pocket, Rasiel pulled out a golden key, that was much like Belphegor's, only around it was a silver chain, and threw it over to Belphegor, who reached up a hand to catch it. "Ushishishishi, of course, fratello; it's the least I can do, for a dead cockroach."