1) What if ...

Error was a student while Nightmare was a teacher in a Nerd and Jock AU :D

Error smirked as he bullied nerds, Such as Ink and Dream. He laughed heavily as he raised his hand, Going to slap Ink but before his hand could touch Dream's cheeks, Someone had stopped him by grabbing his hand.

"Error, My office now!" Nightmare, His history teacher growled as he pushed Error away from Dream, his brother. Nightmare was much more older than Dream by 3 years old. So he was 20 while Dream was 17.

Error looked sternly at Nightmare as he huffed, Pulling his hand away, Glaring darkly at Nightmare. He never liked Nightmare before, Nightmare was a boring, stern and strict teacher for him. Nightmare's favorite student will always be his dear brother, Dream. He always praised Dream in front of everyone and that makes him sick.

"Whatever." He rolled his eyes as he ran away from Nightmare, Going into the wrong direction where the school gate is. He didn't want to obey Nightmare, He likes to being naughty in front of Nightmare but in fact he originally isn't naughty.

"Error." Nightmare hissed as he grabbed Error's arms, Pulling him into his chest. He pinned him to the wall.

Error eyes widened as he blushed a little, Looking at the intimidating teacher in front of him. "L-Let me go!" He stuttered.

Nightmare smirked as he grabbed Error's chin, Making him look into his purple eyes. "No, You are getting quiet naughty these days... Tch tch tch…" He spat as he looked at Error.

Error blushed heavily as he felt his legs melted at the sudden of changes. He tried to look away, But Nightmare's cold and soft hands kept him still. "I-I… S-sr..." He stuttered as he was very embarrassed.

"Oh~ So this is how to make you quiet huh~?" Nightmare purred as he let Error go, Pulling his hands away from Error as he let Error breathed.

Error blushed heavily as he slowly slid down to the floor, Covering his mouth as he looked at Nightmare. Why suddenly his boring teacher turns into a sexy teacher!? "F-fuck you..." Error growled as he looked up at Nightmare.

Nightmare grinned slyly as he bent down in front of Error. Both of their faces were inch apart as Error blushed heavily, Feeling Nightmare breathed in front of his face. "S-sir..." Error stuttered, Looking embarrassed again.

Nightmare giggled as he kissed Error's forehead, "Next time if you are being naughty, You are going to get a punishment~" He purred before walking away from Error, Amused by Error flustered face.

Error squealed in embarrassment like a highschool girl as he covered his face, "S-stupid sexy bastard..." Error growled.

2) What if ...

Error was a siren and Nightmare was a pirate among the seas?

THUD

Nightmare eyes widened as he felt his ship crashed again something. He took a telescope as he looked down, Seeing a siren who was passed out on a big rock. The siren got a colorful tail which was coloured in blue, red and yellow while his ribs were black, Decorated with scars. His face was pretty as well, He had blue tears running down from his eyes as a golden crown on top of his skull.

Nightmare smirked, "Aye, We got a treasure mate. Take 'em to the cargo." He commanded as his sailors, Killer, Cross, Dust and Horror nodded.

They climbed down the ship with rope as they tied Error with the ropes, Making sure it was tight and firm enough so Error won't fall or escape. They pulled Error to the ship as Nightmare grinned at what he had gotten himself into.

"Hmm... Must be worth 17 599 999 G. Er mate, Put 'em to the tank." He growled as Killer dragged Error to the tank which was in Nightmare's room as he threw him into a tank full of water. 

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Error groaned as he slowly grew conscious as he looked around to find himself in a huge tank fish. His eyes widened as he tried to get out, But the tank had a lid on top of it. He gulped slump of fear as he heard laughter outside the room.

The door slammed open with a goopy skeleton with 4 tentacles behind his back walked in with a smug face. "You are awake, Heh." Nightmare smirked as he placed his hand on the fish tank to see the delicate figure he had caught.

Error shuddered in fear as he backed away from Nightmare, Pressing himself to the glass of the corner, Staying as far away as possible from the creepy captain.

Nightmare grinned as he pulled the lid off as the top of tank left opened. Error eyes widened in happiness as his head came out of the tank as he started looking clearly at the pirate in front of him. But his smile turned into a smirk and he sang a siren song where he could control people's mind if they hear the song.

Nightmare's eyes went blank as he motionlessly listened to Error as he slowly sat down. This is one of the hypnotize that sirens like Error could do in defense. He could make the person does anything he want, Like a doll.

"Hehe~ Now tell me your name~" Error purred as Nightmare was completely under his trance right now.

"Nightmare." He answered, Full of no emotions as he stared at Error obediently like a puppy.

"Now Captain Nightmare, Please strip yourself~" Error smirked as Nightmare started to strip himself as all of his clothes fell to the ground. No more clothes were on him as he was completely nude in front of Error.

Error giggled as he watched Nightmare in amusement, "Now-- AHHH!" Error yelled as he felt himself getting hit by something.

He grunted as he completely lose his control over Nightmare as Nightmare back into his sense again. The person that had helped Nightmare was Cross, One of his obedient mate.

Nightmare realized he was completely nude started glaring deeply at Error, Plucking cottons inside his ears so Error's song won't affecting him anymore.

Error shuddered in full fear as he backed away from Nightmare who completely dressed. He was helped by his tentacles after all. Nightmare grinned sadistically as he shoved a tentacle inside Error's mouth, Wrapping a tentacle around Error's body, Squishing it tightly and painfully.

Error gasped in pain but he blushed heavily at the same time, "From now on, You will become my slut and torture toy~ At first I thought I will keep you as collection, But I changed my mind~ From now on you will serve me till your death comes. Understand?" Nightmare chuckled darkly before he started pleasuring Error.

Error moaned helplessly as he cried shamelessly. He wished he hadn't played with a wrong person.