Horror of Horrors!

I guess it would be safer if we open it at your house," you declare, picking up the suitcase.
Will lives a ten-minute walk away. You're halfway there when a cold wind begins to moan in the treetops. Thick fog drifts around you, making it hard to see...and what's that sound? Your head whips around. It's almost as if someone in the fog is whispering, "Thief! Thieffff!"
Will glances over his shoulder. "Do you hear somebody following us?"
"I-I don't think so," you reply. You peer through the fog. You don't see anyone and the voices seem to have stopped. Maybe it's all just your imagination. You keep heading toward Will's. The suitcase is harder to carry. You drop it to the sidewalk, panting.
"Can you help me carry this?" you ask Will.
"Sure." Will picks up one end and you take the other. "It's a lot heavier than it looks."
You gulp. "It wasn't this heavy when we started."
"It probably just seems heavier," Will suggested.
You nod, hoping he's right.
As soon as you enter Will's house, the suitcase feels lighter.
The house is dark. Will's parents are out for the evening. You cross the front hall and enter the living room. You open the suitcase as Will turns on all the lights. Will grabs your arm.
"Look!" he points at the window. A masked figure in a long black cloak stands outside, staring in at the open suitcase.
Did MagnifiCAT follow you?
"He can't get in," Will assures you. "The window's locked."
The intruder raises a hand and makes a gesture. You hear a click, then the window starts to glide open.
You and Will hurl yourselves behind the couch. You peek over the top. Your heart pounds as you watch the window continue to open by itself.
This isn't a trick, this fellow has real magical abilities.
The masked figure leans in through the window. "Give me the suitcase," he demands. "Or else!"
"G-g-give it to him, quick!" Will urges.
What should you do?