A/N: So Halloween is coming soon and I had this idea where the TMI characters decide to celebrate Halloween in the Institute while the Lightwoods are away. I know the first chapter is really short but it's more like an intro than a real chapter. I'll post more very soon, I promise. Also it's kind of an AU. They are still Shadowhunters, Downlorlders, Demons and all but Simon is not a vampire yet, Clary and Jace aren;t told they're brother and sister and it's not during any of the books or events from them, nor between any of them but it's after City of Bones for sure! Everything else is the same. For now, enjoy :D

Disclaimer: I guess you won't believe me if I tell you I own anything except the plot so all I could do is admit that I don't :D

"Are you sure about that?" asked Simon for about fifth time since his best friend told him about the evening. "You know that Jace-"

"But Isabelle is the one that invited us. And Jace doesn't have the word as she said." Interrupted him Clary, trying to assure him everything would be okay.

"That won't stop him from kicking me out, you know" muttered the boy quietly.

Clary only rolled her eyes at this.

"I promise you he won't do anything like this to you" she said.

"That's what you're saying" wouldn't give up Simon.

"Oh, come on, Simon! It's Halloween! It would be a great fun. Just relax"

"Why do I always hear that before something totally not-fun happens?" he asked sarcastic. "And stop rolling your eyes, you know I'm right." He added as Clary was about to do it again.

"Let's just go and see how it's going to be, okay?" she asked him "Besides, Luke promised he'll pick us up at anytime if we want to go home earlier."

"Okay, okay. But hey, don't forget I warned you about this."

"And don't forget, we'll be away for only three days so try not to set the Institute on fire, right?" said Maryse for a last time before she and her husband, Robert, leave with their smallest child, Max, for Idris.

"Yes mom, don't worry for us. We'll be just fine" said Isabelle as she smiled to her mother like an angel.

"Unless someone decides to make a Halloween party and invite half New York" added Jace, smiling mischievously, which brought him a death glare from his adopted sister.

For a moment it looked like Maryse was about to say something but Robert interrupted her.

"Oh, come on, Maryse. Leave the children have a little fun. I'm sure they won't do anything stupid or reckless." With those words, he dragged his wife along with him to the car, with the already waiting Max.

"Bye" said the others in unison, just waiting them to leave. As soon as this happened, Isabelle turned her head to Jace, her eyes burning holes at him.

"Are you nuts?! What are you thinking by telling them this?!" she asked with an angry voice.

"And what exactly are your thoughts, Isabelle?" asked her brother, Alec, glancing at her. "How do you think they will react if they find out you've decided to throw up a Halloween party while they're gone?"

"Oh, just calm down Alec, we're sure Izzy got the situation under control. Like every time when she has a crazy idea" said Jace with sarcasm.

"Of course everything would be under control." Told them Isabelle, calmly. "Besides, those are only a few people and we all know them already. Clary would be here, too." She said.

"Yeah, with her mundane friend." Said Jace, frowning.

However, Izzy just decided to ignore that.

"Magnus Bane would also probably come" she added, looking at her brother pointedly.

"What?" he asked maybe a little too quickly. "What is he going to be here for?"

Isabelle's smile just went wider as she once again said.

"It would be fun, I'm telling you"