Dipper, Pacifica, Sandra, and William soon arrived at the shack, and were greeted by a surprise. Stanley stood in the doorway, grinning from ear to ear.

"Welcome back guys," Stanley said. "I have a surprise for you all."

"You're going to start wearing pants around the house?" Dipper asked, making Sandra laugh.

"No," Stanley replied, giving Dipper a glare. "I'm throwing a party tonight to make some money from these suckers in town. The family gets to come free, and so do their boyfriends/girlfriends. It's going to be tonight at eight."

Mabel soon appear by Stanley's side, grinning.

"And I'm decorating!" Mabel said.

"Oh boy," Dipper mumbled.

"Hey Sandra, I'll see you tonight, I gotta go," William said, kissing Sandra's cheek. "Love you."

"Love you too," Sandra said, and waved as William walked off.

"Now, come in guys," Stanley said, leading everyone in the house.

Mabel stopped when she saw something on the ground. She picked up the item, and examined it. In her hand, she held William's sunglasses. She looked up to see William's figure quickly disappear behind the horizon.

Without thinking, Mabel took after him, the sunglasses clenched in her hand. She followed him right into town, where he immediately walked into his house. Thinking that this was an opportunity to spy on her aunt's boyfriend, she ran to the closest window, and peeked in.

Inside, William removed a sheet that was covering a crystal ball. A picture began to form in the ball, and it was of Sandra. She seemed furious, and was surrounded by white fire. She seemed to be screaming something, but Mabel couldn't hear what she was screaming. William smiled, and began muttering to himself. The picture slowly faded away, and William covered the ball back up with the sheet. He then stretched, seemed to say something, and began to glow. Mabel watched, with wide eyes, as William's shape began to change. The glowing disappeared, and William had changed into a familiar figure. Mabel gasped, and quickly covered her mouth. As if he hear her gasp, William turned to the window, just as Mabel slid out of sight. She couldn't believe it. All this time, William had been him. Tears began to form in Mabel's eyes, as she left the sunglasses by the front door. She took off running to the shack as fast as her legs could carry her. She had to warn Sandra. She just had to.

***

William changed back to his original shape as he opened his door. He took a step outside, and was greeted by a soft crunch. He looked down in surprise, and picked up a pair broken sunglasses. They were his sunglasses. He frowned, and his frowned deepened when he smelt an all too familiar smell. Glitter. Mabel was here, and she had probably seen who he really is. William's frown turned into a sly smile. He was going to tell Sandra tonight, and thanks to Mabel, she just furthered his plan. He placed the sunglasses in his pocket, and formed a new pair in his hands. As he put them on, he knew that today was going to be his day.

***

Sandra was just about to go outside to hand out flyers for the party, when Mabel burst in, crying.

"Mabel!" Sandra gasped in surprise, almost dropping the flyers. "What's wrong?"

"It's William," Mabel replied, hugging Sandra. "He's...he's..."

"Hey, hey, hey, calm down Mabel," Sandra said, patting her back. "Why don't you relax in the living room while I pass out the flyers. I'll be back soon."

Sandra unhooked Mabel from her, and went out to deliver the flyers. Mabel stood there, trying to calm down. Dipper walked in, and looked at his sister confused.

"Mabel?" Dipper asked, concerned. "Are you okay?"

Mabel grabbed her brother by the shoulder, and whispered in his ear what was bugging her. Dipper's eyes grew wide as she explained everything.

"No...it can't be..." Dipper whispered.

***

Sandra stood by the doorway of the shack as the moonlight made her black and golden dress sparkle. Guests were already entering the building, but William was late. She felt someone tug her arm, and she looked at Mabel. Her pink dress sparkled, and so did her pink heels. Her makeup outlined a sad face.

"Mabel?" Sandra asked. "What's wrong?"

"It's William," Mabel replied. "He's not who you think he is."

"What do you mean?" Sandra asked.

"Sandra!"

Mabel and Sandra adverted their gaze to see William, in a black and golden tux. Sandra waved at him, and he came running.

"Be careful, Auntie," Mabel warned Sandra, causing her to look at her niece. "William is not who he seems."

And with that, Mabel moved off into the crowd, her pink heels glittering as she merged with the crowd.

What did she mean by that? Sandra thought, just as William arrived.

She dismissed it though, as William took her arm, and led her away.

***

Mabel searched for her brother, and found him sitting with Pacifica, Candy, and Grenda. They were all wearing what the wore to the Northwest Party on the night of the haunting. Dipper had apparently told the girls about William, because they looked at Mabel with concern.

"Did you tell her yet, Mabel?" Dipper asked worriedly.

"I didn't get a chance to, but I did warn her about him," Mabel replied. "I just hope he doesn't harm her."

"From what Dipper told us, he probably will," Candy said, making Mabel's frown deepen. "and he seemed like such a nice guy."

"Let's just have things play out tonight, and let's see what happens," Grenda suggested.

The others nodded in agreement.

***

As the night went on, the party dwindled down until it was just the Pines, Candy, Grenda, Pacifica, and William. Stanley and Stanford left a spotlight on the dance floor, and it was the only lit light. They were in the basement discussing something, and the others were upstairs, discussing what they were going to do with William. Sandra and William were still on the dance floor.

"May I have this dance?" William asked, holding out his hand.

William smiled as Sandra looked at his hand. Sandra uncertainly took it, remembering what Mabel had told her.

Be careful, Auntie. William is not who he seems.

William led Sandra into the spot light, and they began to dance. William noticed Sandra's nervousness, and frowned a little.

"Something wrong, Sandra?" William asked her.

"Huh? Oh, yeah...I-I'm fine..." Sandra stammered in reply.

"Sandra, please tell me what's wrong," William said.

Sandra bit her lip before she decided to ask him.

"Who are you really?" Sandra whispered.

William laughed softly, and leaned in towards Sandra.

"Welcome back to Gravity Falls, Sandra Pines," William whispered in her ear in an all too familiar voice. "Let's congratulate that with the 'Dance of Death.'"

William tightened his hold on Sandra's hand, and made her spin. Sandra screamed in surprise, and William grinned. William finally caught Sandra and pulled her close. Sandra watched in shock as William removed his sunglasses, and his eyes turned yellow with black slits.

"I-It's...y-you," Sandra whispered in shock.

"Oh Sandra, you're too kind," William said, letting Sandra collapse to the floor. "Of course I'm Bill. And thanks to you, my plan is almost complete. I might leave you alive, but only because I'd love to torture you more."

With that, he disappeared. Sandra layed on the floor, tears rolling down her face. She put her head in her hands, and began to cry softly.

What have I done?