The Doctor stared blankly at him.
"Oh, hello! Have we met before?"
He looked at his back, and gasped.
"I'm a Pegasus!!!"
The thing laughed, becoming more and more demon like every second, as it's fork tongue increased in length and spikes started to appear on its body.
"I never realized how fun post-regeneration side effects ae from the outside..."
He slithered around the Doctor, two bat-like wings now sprouting, as his overcoat became bone armour.
"Dear Doctor, you are so helpless right now, killing you would be too easy. I've waited so LONG to escape YOUR WRETCHED MIND," the words he put emphasis on sounded like they were spoken by a gigantic spider, a hiss rolling with each one.
"YOU'VE MANAGED TO KEEP ME THERE FOR AS LONG AS YOU LIVED, BUT IN THIS PLACE I'M FREE TO ROAM, TO FEED," he stared at Darts, his eyes now glowing like fire and two small horns extension from the back of his skull. His wings had extended, and would soon be able to support him fully.
"TO KILL."
He jumped for Darts, but dissipated into smoke upon impact.
"Oh, my dear Doctor." even his normal voice now had some sort of demonic tune to it.
"This... Slendermane FREED ME, yet I know so little about it. I really should pay it back. How about we add a bit of fun to its game...?" while he talked, his shilouette appeared to float in the smoke around them.
"I'll be waiting DOCTOR."
The smoke vanished into the Dark corridor revealed by Darts with a laugh wich filled the empty halls. Darts turned back to the Doctor.
He was completely dumbfounded.