A/N: I am fully blaming Crimson Priestess of Miracles for this little brain worm that refused to leave my head. After reading her piece of The Embrace of a Reunion, a thought of a Dark Signer AU wiggled into my head. Though, Ren's not a Dark Signer. Instead, he's a demon...because of Hazbin Hotel/Helluva Boss (if you do not like violence/dirty jokes/harsh language, do NOT look that up as both are mature indie animations and not meant for kids). Also taking a bit from Blue Exorcist for one bit where the tail is a demon's weakness (though uh...maybe not entirely in the same context with this piece; especially if I somehow write a continuation of it that definitely would have to go on AO3). Well, enjoy this little piece!

Disclaimer: I do not own Yu-Gi-Oh! 5D's. That goes to the respective owner(s). I only claim my OCs. And there's language and 'hints' at other things (obviously nothing shown as to keep 'T' rating and censoring of some words due to rating too). Also, this has been inspired by Crimson's The Embrace of a Reunion which I highly recommend you check out (which is a lot better than what I threw together)!

Underworld AU- Mischief:

(Yusei's POV)

I walked through the halls, trying to find any sign of the little mischief maker. After the yelling I had just gotten from Jack, I figured it'd be better if I found him first. Ren had decided to 'redecorate' the throne room while everyone was out. Compared to his other pranks, it could have been much worse than just mixing up where everything was.

As I was about to round a corner, I caught movement from the corner of my eye. Turning, I found a long black tail with messy fur at the end flicking down. Trailing the source came to the very one I was looking for. He was laying on one of the rafters, arms folded behind his head and one leg dangling off the side. I smirked. Bingo.

Stepping up, I kept from getting his tail flicked into my face so as to not alert him that I had found him. Then, while his flicking was at a lull, I grabbed it. He shrieked, yanking his tail out of my grip (I kept it loose to not hurt him) and losing his balance. He just barely managed to grab a hold of the wood with his nails. "What the f***?!"

"Language, Ren," I responded calmly. He looked down, seeing that I was the one to grab his tail.

"God dammit, Yusei. Don't sneak up on me like that!" he huffed. He jumped down, keeping his tail curled toward him.

"Just be glad it's me and not someone else. I got alerted about your latest project." I went over to the wall, sitting down with my back against it. "Now what could have possessed you to turn the throne room upside down?"

"Maybe because I'm in a pissy mood?" I gave him a 'really' look to show that wasn't enough of an answer. He grimaced, trying to shrink in on himself with his arms crossed. "Fine, some guards were talking how I'm only here to be a w**** and I should be lucky I'm here out of the goodness of the Dark Signers' hearts and all that shit."

I sighed heavily and patted my lap. He hesitated a minute before caving and sitting down. "So your way of taking it out was to make Jack mad instead of coming to someone about it?"

"Well...um..."

"Ren, I told you. If the guards or soldiers or anyone here say anything like that to come to any of us to sort out. Not take your frustration out by causing mischief. I'll talk with Jack later and tell him the cause of your prank."

"Mmph, fine." He curled into me and I wrapped my arms around him. "I'll try better next time."

"Good boy." I stroked his hair, avoiding the horns poking through his dark brown hair. I let him have a moment of calm before I reached my hand to near where his tail connected to his body and gripped it hard.

"Ah!" He jumped a bit, his cheeks bright red. "Wh-What was-" His blue-green eyes met mine as I gave a dark smirk.

"Don't think this completely gets you out of punishment, though."

A/N: ...I think I'm really pushing the rating at this point. ' Like I said, I don't put anything expletive as it's not something I write and also the biggest reason for writing a story I think would fit an M rating on here would be because of violence and language (I mean, have you see how much I have to censor with Ren?). If I ever do write more, I'll just put it on AO3 as it'd get removed on here. But uh...don't hold your horses that will happen anytime soon from me. I was getting embarrassed just writing this and the closest thing was the a** grab and the last line here. Guess all that watching of Hazbin Hotel and Helluva Boss did squat for me XD

Also, this isn't a complete Dark Signer AU since only the main characters of YGO 5D's are Dark Signers and Ren's...well, not. He's kinda of an ordinary guy (barring certain Pokémon crossovers and there was an idea with an old one that Ren faces a DS version of himself that I never really wrote much on/have thrown out the window) for the most part, so I got creative to explain why he's there. I figured maybe a soul could get corrupted into becoming a demon and thus explain how Ren exists in this AU. And the Blue Exorcist tie is mainly because of the voice I imagine Ren having (Bryce Papenbrook).