Note: I don't own anything, Star vs the Forces of Evil belong to Disney and Sonic belong to Sega. Also, this is my very first Fanfiction, so I'm very new to this so please understand. With that out of the way I hope you enjoy the story but do expect to see more notes in the future. This story takes after the events of Star vs the Force of Evil season 4. Now let's began.

Sonic and Star in Adventure in Mewni

Prologue

On Mobius we find Dr. Eggman launching a full-scale attack on Angel Island, a floating island in the sky, firing missiles from his Egg Mobile, Eggman's personal all-purpose hovercraft along with a bunch of Buzz Bombers and other flying robots to set parts of the floating island ablaze in fire.

"Come on Knuckles" shouted Eggman as his robots continues to bomb the island's jungle with bombs and lasers. "Come down from your precious alter and save your island in the heaves."

Suddenly a biplane with a red fuselage and two grey main wings flew beside pass the Egg Mobile, piloted by anthropomorphic yellow twin tails fox with a blue hedgehog and a red echidna on top of the plane's wings.

"Sonic, Tails, Knuckles" shouted Eggman as he secretly whispered in a radio and smiled "phase 1 complete, starting phase 2, you may begin."

"Eggman" shouted Knuckles the Echidna, the guardian of the Master Emerald and Angel Island and solo surviving descendant of the Knuckles Clan.

"This is another one of your schemes to steal the Master Emerald?"

"Of course, isn't it obvious, I want the Master Emerald for my new plan, hence the missiles and robots."

"Attack" as ordered, the robots turn their attention to the biplane and attack as Eggman fired missiles at our heroes.

Tails, who pilot the biplane named the "Tornado" managed to maneuver the plane to avoid the missiles and lasers as he said, "Woah this time he means business."

As Tails use the Tornado's built in BB guns to shoot down some of Eggman's robots, Sonic the Hedgehog, the fastest thing alive, just grin and said, "he not holding back this time, all right guys, ready?"

"Ready" shouted Tails and Knuckles, then sonic replied, "ok let's do this."

Tails turn the Tornado towards a squadron of robots and began shooting BB bullets at them while Sonic jump of the wing and use his homing attack to destroy some robots to reach Angel Island safely, while Knuckles simply glide to the floating island while at the same time our heroes avoided missiles from the Egg Mobile's missile launcher.

"Hey Eggman, take this." Tails began shooting at the Egg Mobile which forced Eggman to move to avoid taking more hits, as Sonic and Knuckles safely reach the island, they felt a tremor and then Knuckles realized "oh no, the Master Emerald, someone took the Master Emerald and without it, Angel Island will fall into the ocean below."

As Knuckles said, Angel Island began to slowly fall from the sky towards the ocean which Tails and Eggman noticed from the sky.

"Hehe he, phase 2 completed now on to phase 3." shouted Eggman as he piloted his mobile towards the Master Emerald Altar which is located at the very center of the island.

Tails put out his communicator and contact Sonic and Knuckles.

"Guys, I think the attack is all just a distraction, Eggman must have got someone else to get the Master Emerald."

"We already know buddy" replied Sonic as he and Knuckles ran towards the altar, "we're on our way to the altar. meet us there."

"Roger that"

"You think Rouge is working with Eggman?" asked Knuckles "after all, that bat has her eyes on the Master Emerald for a while."

"I don't think so, she works in the Guardian Units of Nations remember" answered Sonic as he and the Echidna race towards the altar. "Although she does steal jewels, besides that, I think it's Fang and his gang Bean and Bark. Those Hooligans often work with Eggman more than Rouge, so it makes more sense."

When Sonic and Knuckles reach the altar, they saw a group of humanoid lizards, arm with swords, spears and shields. they lizards are carrying a giant diamond shaped, green gemstone down the steps of the altar as one of them noticed Sonic and Knuckles and yelled, "Intruders, to arms and stop them for Lord Seth." As told, some of them drew out their weapons and charge at Sonic and Knuckles.

"Well, this is unexpected" said Sonic as he uses his super speed to knock out some of the mysterious lizard warriors "who are these guys?"

"Don't know but they won't get away with stealing from the Guardian of the Master Emerald" shouted Knuckles as he punches a few of the warriors.

As Sonic and Knuckles fight their way to save the Master Emerald, Eggman in his mobile flew over them and push a bottom to put out the mobile's built in BB guns and began to shoot at our heroes, protecting the warriors as a strange portal open in front of the warriors carrying the giant emerald.

"No, the Master Emerald," shouted Knuckles as he and Sonic can only dodge and defend their selves as they watch the Master Emerald disappeared through the portal as a warrior shouted, "we got what we came for, all Septarian Warriors retreat through the portal." while Eggman continues to fire BB bullets at Sonic and Knuckles which forced our heroes to take cover behind a large rock. Luckily for them, Tails flew by in the Tornado and shoot the Egg Mobile with the plane's BB guns forcing Eggman to fly towards the portal as the last Septarian went through.

"Thanks for the Master Emerald Knuckles, until we meet again" shouted Eggman as he piloted his mobile through the portal, seconds later the portal closed just as Sonic ran from cover to the spot where the portal was, As Angel Island slowly land into the ocean. "I'm sorry Knuckles, they got away with the Master Emerald" said Sonic as Knuckles ran up beside him.

Then Sonic's communicator lit up and pick up to hear Tails' voice.

"Guys, I may know where Eggman and those warriors went to, I'll pick you up at Launch Base Zone and we can go to my workshop".

"Ok little buddy" then Sonic turn to Knuckles and asked, "ready to get the Master Emerald back?"

"Always" answered Knuckles as they rush to Launch Base Zone which is located on Angel Island.

3 hours later at Tails' workshop, Sonic asked "do you have something to find Eggman and the Master Emerald?" which Tails explain as they walk into the workshop.

"You bet Sonic, I managed to scan the portal before it closed, and I can use that scan data to locate them with the help of my new invention" then Tails put out a device that look like the Miles Electric pad but with a black rectangle appearance with two blue-glowing antennas on top and a cyan colored touchscreen in the middle. "I call it my Monitor Electronic Dimensional Emerald Locator or M.E.D.E.L for short." then Tails open a deck drawer and pull out a red gemstone.

"Whoa! A Chaos Emerald!" said Sonic then Tails continued "yep I found this in Mystic Ruins and use it as a power source to my device, you see I design it to allow me to open portal to anywhere in any dimension. recently I located the 5 remaining Choas Emeralds on a world the M.E.D.E.L ID it as New Mewni, but I could get an exact location because a strange energy signature. If I'm correct..." then Tails plug in a flash drive containing the portal scan data to M.E.D.E.L and after 30 seconds Tails spoke again as Sonic and Knuckles watch the screen which show New Mewni energy signature and the portal energy signature side-by-side

"Yes, these two signatures matched each other perfectly."

"Great work Tails" complemented Sonic and then Knuckles cut in "but Tails didn't you say you couldn't get an exact location of the remaining Emeralds and since the Master Emerald is more powerful them the Chaos Emeralds why not locate the Master Emerald itself?"

"True, but with this Emerald in my hand, I could still open a portal to New Mewni to the closest possible location of one of the Emeralds, and that was the idea Knuckles, since the Master Emerald has a larger power then the other Emeralds but for some bizarre reason, I couldn't locate the Master Emerald. Plus, Eggman could have place the Master Emerald in a place where its energy is being jammed to prevent its location from getting discovered."

"You did your best Tails, at least we know where Eggman went and those Septarian Warriors come from, no doubt Eggman is going to use the Master Emerald to locate the remaining Emeralds?"

"That does sound like Eggman, but those Septarian Warriors, how do they fit in it?" asked Knuckles "and who is this Seth character?"

"Only one way to find out, are you guys ready for another adventure and save the day as usual?" asked Sonic which Tails and Knuckles replied with a thumbs up.

"Then let's go" said Sonic as Tails place the red Chaos Emerald in a container connected behind the M.E.D.E.L, then type in the coordinates and using the Emerald's mysterious energy, which allow Tails to open portal just like the one at Angel Island and Sonic, Tails, and Knuckles went through for a whole new adventure.

I know it's long, but this is the prologue and I'm new to writing fanfiction, no one is perfect. Again, both Star vs the forces pf Evil belong to Disney and Sonic the Hedgehog belong to Sega. There will be longer chapters, but it will be worth reading I promise you that. Chapter 1 is where Sonic and his friends meet Star Butterfly and Marco Diaz.