Birkin in Equestria

William lied on the floor bleeding heavily from his shoulder.

'Is this it? Has my time already come?' he thought. The questions were endless. Then suddenly he felt something inside him react violently to him receiving physical trauma. The pain began getting worse by the second and it was beginning to focus on his right front hoof. The pain was becoming unbearable and he felt his vision beginning to blur. Just before he passed out he let out a bloodcurdling scream that turned deeper and more distorted until he blacked out completely.

Tirek and Discord entered the throne room where the princesses were lying on the floor seemingly exhausted. Tirek made an attempt on sucking them dry of magic, but was surprised to find that they had no magic at all.

"Where is your magic?!" Tirek demanded angrily.

"We're not telling you. And we never will." Luna said firmly.

Tirek gritted his teeth in anger as he charged up his magic to put an end to the three princesses, but just as he was about to fire a big fleshy claw dug into his back and made it's way through his body until it came out of his gut. He turned his head to see a gruesomely mutated William standing there with what was once a hoof piercing him. The hoof was covered in muscle tissue and a giant eyeball was looking out of the shoulder. The stallion twitched like crazy and cried out in pain multiple times. William then pulled the claw sideways, ripping out over a third of Tirek's insides out. Blood and organs spilled all over the floor, everything within six meters radius was painted red. The amount of flesh ripped was too much and Tirek's upper body fell right off his lower body. He was 100% dead. Magic flowed out of his mouth, flew into the sky and then took off in almost every direction possible.
Meanwhile back in the throne room everyone was looking at William with eyes at the size of pinpricks. The princesses had seen bad things before, but this easily took the cake. William was standing in the middle of the carnage while gasping for breath. He soon let out a scream of agony that echoed through the halls of the castle. He grabbed at his temple as the mutations seemed to retract. The giant eyeball on his shoulder closed and disappeared, the claws retracted into his hoof and his colours began to return. Before the mutations fully disappeared he managed to force out five words through the pain.

"Somepony...help...me!!! It...hurts!!!" he screamed before he passed out and the mutations disappeared completely.

The princesses looked at the passed out unicorn with expressions mixed with fear, shock and confusion. Never in their lives had they seen anything like this. Celestia got up on her hooves and carefully walked over to the unicorn. She stopped every time he twitched to stay safe, but nothing happened. When she came over to him she bowed her head down and she heard him mumbling something. It was hard to make out, but she heard one sentence clearly.

"G is my creation."