Disclaimer: I do not own Invader Zim, The Grim Adventures of Billy & Mandy, or any of the following characters.
Prologue
Squashed between Billy and Mandy on the former's couch, Grim had been lulled by the sounds of the B-grade horror film they'd stumbled upon while flipping through channels; the flickering light of the TV, casting shadows on the walls.
"Grim..." Mandy handed the skeleton her empty glass. "Refill."
"Why don't you get it yourself?" Grim chanced, knowing it was the wrong thing to say, when Mandy sent him a death glare that put his title to shame. "Oh, alright. Whattaya want?"
"Dr. Pepper. Hold the ice."
Grim reluctantly snatched the glass from Mandy's hand and shuffled toward the kitchen, cloak tail dragging behind him. "Why do I let a little girl boss me around like that?" He muttered to himself, opening the fridge and surveying its contents. "I'm the Grim Reaper, for cryin' out loud." As pathetic as it sounded, though, it was easier to understand if you knew Mandy. Her own parents were terrified of her. "Here's your—" Grim nearly spilled Mandy's drink upon returning to the living room. "Billy, wait...! Don't take me scy—!" Before Grim could finish his sentence, Billy had already disappeared into the swirling, green vortex he'd opened; igniting very real flames in Grim's eye sockets. "If you didn't have me scythe, I'd let you fend for yourself, wherever you are!" He yelled into the portal.
"We'd better hurry, before it closes."