The Grim Adventures of Billy Mandy are owned by Maxwell Atoms/Cartoon Network Studio.
Avengers: Infinity War is owned by Marvel Studios.
Ok so bear with me, thought process for this story was that Grim is one of many, a legion actually, of reapers scattered around the cosmos in charge of ferrying the dead. He has his mom and for this story Death from Marvel Comics is his sister.
Enjoy--.
Thanos covered his face from the blast the witch had created. It was an impressive sight and he could empathize with her pain, having sacrificed Gamora not too long ago. He took a step forward, only to stop.
A green mist appeared and begun swirling about and crashing into the ground, creating a hazardous light beaming into the sky. It dissipated, and in its place a shadow grew and elongated. The shape it took was a shifting thin black mass over 6 feet tall. As it took a more substantial form, black robes appeared, Skeletal hands dropping down from within the sleeves and a skull protruded from a newly formed hood.
"Reaper," thanos greeted, "I am surprised one of your kind would grace me with their presence, especially with my impending victory," he smirked "especially when you've all been silent so far."
The skeleton shifted his gaze upon the king, then turned again to survey the surrounding destruction. Crossing his arms as he continued, "to be honest, I'd rather not have had to make an appearance but it would seem that my hand is... forced."
"Forced?" Thanos inquired, "what about my actions would be considered severe enough to force YOUR hands? Considering all I'm doing is celebrating your profession?"
Grim clicked his teeth, his stare resting upon a woman in red kneeling near a faded corpse, silent tears falling down her face as she watched with caution at the newcomer.
"Celebrating?" Grim repeated in a mocking tone, "of course you would see your actions, as absurd as they are, to be equal to that of our mission."
Thanos chuckled, "isn't it? I am bringing you countless souls to reap, all for the betterment of the galaxies, of the universe."
"We do so following the NATURAL ebb that is death," grim spat out turning to face Thanos, "your actions are nothing more than typical of one spurned, lashing out due to an unwarranted sense of entitlement."
Thanos glowered, "you speak out of turn reaper. The past aside, I do what I do because there is no other way." He took another step forward, his left hand clenching within the gauntlet. "And if it should prove my worth to your sister, then that would be a bonus"
"You should be more careful Grim," he warned, raising the gauntlet and displaying the infinity stones he possessed" considering the power I now wield."
Grim raised his own hand, materilaizing in front of him a solid black staff, a blade suddenly appearing on the end.
He gently grasped the scythe, "even with the power of the stones, you would still be but a mad titan. You would find yourself hard pressed in fighting me"
Wanda could do nothing but witness this exchange. She already had a feeling of the titan's insurmountable power, but what she felt from the skeletal figure, dubbed Grim from Thanos, was suffocating. It felt cold and dark, as if she was in a lightless cave far beneath the surface with no way out. She couldn't tell which power was greater, but she knew she would see the two clash soon.
She would have, until another portal appeared directly in front of her. It was the same sickly green vapors as before, except it swirled to form an archway where a figured stepped out.
It was a young girl. A young girl in a pink dress wearing black mary jane shoes. Her blonde hair accessorized with a black headband giving the appearance of two little horns.
"Bonehead," a deep and melancholic voice spoke, "I thought I told not to leave the city unless given permission."
"I did," Grim grumbled, "billy said I could."
"And I could have sworn that I said that billy doesn't count due to the fact that he is an idiot."
"Fine, I dont have permission but this is important to me. And it should be important to you as well Mandy, if you wish to keep living.
Mandy narrowed her eyes at grim, "you know you're not allowed to kill me, you're not even allowed to kill Billy."
Despite speaking calmly, the underlying anger could be felt.
"Dont think you wont be punished for this. I've been meaning to give Saliva a new bone to chew on. Your femur would be perfect."
"Not me child, I'm referring to this titan before me," grim replied, shivering at the thought of surrendering his leg bone to the mangy dog.
Thanos stared in disbelief.
"I had heard rumors that one of your own had become enslaved, forced into unending servitude of two mortals, children no less," he chuckled, "to think it would be you is a delightful surprise. Tell me, what possibly could have happened to place you in such a shameful stature?"
Wanda couldn't hear Grim's reply, only vaguely hearing muttering. She could have sworn she heard something about limbo and cheating and a hamster.
If she was confused before, she was bewildered now. All she could do was look from Thanos to Grim
Mandy shifted her scowl towards the massive being in front of Grim.
"Grim," she asked, " Who's the giant raisin?"
"Just a creepy stalker after my sister. She's not interested. Last I talked with her she had a thing with some dead pool or something"
"And he's trying to kill half the known universe?" She asked, still meeting Thano's gaze with her own calculating stare.
Grim answered, "That's his plan, unless I or others can put an end it."
Mandy inquired "I didn't even know you had a sister, I've met your mother sure, but you never mentioned a sister."
Grim shrugged his shoulders, "technically she's my sister. Its...its," he struggled to explained, "its cosmically complicated, let's leave it at that."
"But he's still creeping on her?" Her brow raised in silent judgement.
"You have spirit child," Thanos interjected, "I would be curious to see how you develop should you survive."
Mandy ignored Thanos, instead turning to Grim, "SHOULD I survive?"
"50/50 you turn into ash if he succeeds" he answered.
Mandy thought for a second, before turning back to the portal. She paused briefly to take note of Wanda.
"Fine, clean up this mess. After you're done, come back home and we can get ice cream. If there's time maybe I'll let you watch your soaps for the evening"
And with that she disappeared into the swirling green portal, shortly dissipating after her departure.
Grim waved his scythe, "oh ho ho ho ho You heard her," he twirled the scythe, stopping it and grasping with both hands, "I've got me some ice cream and soaps to watch after this."
He narrowed his eyes and tilted his head, giving a sinister grin, "now then, where were we?"
Wanda could only stare as the reaper raised his scythe and lunged forward, laughing maniacally as the blade sang through the air.