Quite a quick opening, but here you hit the MEAT OF THE STORY! MUWAHAHA! Take it vegetarians! lol jk
I do not own the Japanese Franchise called Kamen Rider

Kamen Rider Ryuki VS AGITΩ
Mirror War

Chapter 2 – A New Fight

Driving down the road Shinji thought about the current status of the Rider War, not counting the mysterious Odin or himself there were only five other riders left in the war. At this point with all the tension, Shinji was beginning to worry that maybe he really couldn't stop the war.

He shrugged the thought away and was about to turn into Atori when his phone rang, he quickly picked it up "Hello..." putting it back down he took his helmet off and brought it back to his ear "Hello...ah Editor-in-Chief."

"Hey Shinji, I've sent Reiko-san out to cover another missing person story...this one is much more confusing than any of the ones before."

Shinji raised an eyebrow "Much more...confusing?"

"Three entire families have disappeared."

Shinji's eyes widened, three entire families, no doubt taken into Mirror World to be eaten by the monsters, it was drastically different than the one by one disappearances that have been plaguing the city.

"I'll go join Reiko-san." Shinji hung up and was about to turn when another cycle pulled in next to him, taking off his helmet the man looked around.

"Such a nice place...and small at that...is this a good restaurant?"

Shinji turned to the man who was smiling and looking around "Yes...but don't order coffee, Obaa-san only serves black tea."

The man laughed at that and held out his hand to Shinji "Thanks! I'll remember that!"

Shinji nodded to him and drove off.

Breathing heavily Kitaoka Shuichi leaned against a wall and fell into a sitting position across from Asakura Takeshi "I still...can't...believe...that you..."

"Drop it...I'm only doing this...so I can kill you later...remember that!" Asakura coughed and turned away.

Before Kitaoka could comment he noticed Asakura was looking at another rider "How easy it would be to take you both out now...I enjoyed watching your battle against those Mirror Monsters...I would've been a highlight of my life to see you die to those pathetic fools rather than another rider."

Kitaoka clenched his teeth "Sudou..."

The murderer just grinned and turned to Asakura "I've enjoyed killing various people with my new power...but you have done so without being a rider...something I admire Asakura..."

He walked past them laughing to himself. If anyone knew Asakura Takeshi as a sadistic serial killer, they would prefer to be near him, than Sudou Masashi.

Shinji stood with Reiko outside of a family home "So strange...could this really be another set of mysterious disappearances?"

Shinji turned to Reiko who sighed "Reiko-san...do you think a person is behind this?"

"While it may be as impossible as the rest...it doesn't follow suit with the rest..."

As she continued Shinji turned quickly "I'm not going to let it happen again." He ran to his bike, got on and drove off as Reiko called over to him.

Shinji got to the site and quickly ran up to the home's window, he looked inside as a person was dragged into a mirror "Another family..."

Quickly, he pulled out his Ryuki Deck and held it to the window, a belt appeared in space and flipped over to attach to his waist, he threw his left arm across his chest and yelled "Henshin!" before sliding his deck into place, there was a glow as mirror images of Ryuki fused with him.

"Alright, here I go!" he yelled before jumping into Mirror World.

He landed and saw three Dispiders all turn to him, the beasts all shot out webs and he dove aside, drawing out a card and sliding it into his Drag Visor. -SWORD VENT-

As he jumped out to combat them a Wildboarder ran right into him and knocked him over, looking at the boar-like beast then back to the approaching spiders, Ryuki put a hand on his forehead "Mirror Monsters...together?"

He dove aside as the boar charged again then ran at the spiders, he jumped into the air and landed on one's back before stabbing it, as it roared the other two launched at him, he jumped away and they smashed into each other before falling in a pile.

As he was about to pull out a card, two swords flew through the air and hit him, he fell to the floor and looked to see two Zenobiters approaching, they caught their weapons and ran for him.

Quickly Ryuki moved aside as the boar ran past him and into a Zenobiter. Ryuki charged at the other and as the swords clashed, he kicked the monster away. As it stumbled he ran in and ducked below another swing before tackling it to the floor.

As he rolled over he pulled out another card, two of the Dispiders managed to stand and shoot webs at him. They caught him and he was stuck to a streetlamp "This is not good!"

As they neared, one of the Dispiders' heads snapped back as if it were punched, as it fell over the other was bound and swung into the air, it was released and flew into the Wildboarder that was charging towards Ryuki.

The dragon rider was cut loose as Verde appeared "Well, well...look at you...you are as pathetic as you fight."

"What? That doesn't make any-" Verde punched him and turned to the monsters, he pulled out a card and put it onto his clip as it retracted into his thigh -ADVENT- Biogreeza appeared next to him and shot out its tongue.

"Get your Final Vent ready..."

As Ryuki loaded the card Biogreeza wound the Wildboarder and threw it into the Dispiders, Verde charged at the third Dispider which regained its footing and the two Zenobiters.

-FINAL VENT- Dragredder roared and flew around Ryuki, he jumped into the air and flipped over, bending one leg back and extending the other before yelling.

Dragredder let loose flames that covered Ryuki, he shot into the three fallen monsters and they exploded with their energy being eaten by Dragredder.

Verde back-flipped over to Ryuki, the red rider looked at him "What is this all about? Why are you helping me? Why are the monsters-"

Verde punched him again "Now is the time to get away!" Ryuki attempted to punch him back but Verde moved aside "Look!"

Finally turning around, Ryuki noticed Gelnewts and Deadlemurs appearing, a massive group of about 30, followed by several more boars,

"Why are there so many?" -FINAL VENT- An echo rang out and the monsters scattered as a black missile shot through the group and cleared at least half of them away in the explosions, Knight Survive turned on his cycle "Get out of here!"

Ryuki turned to Verde but the green rider had escaped already.

Outside, Shinji rolled across the ground and turned "What was that all about Itsuro?"

The millionaire rider shrugged "I hate you Kido Shinji, let's get that straight right now...but it's because I hate you that I want to be the one to put you out of your misery...get real here...I can't just stand by and watch you die THAT pathetically."

Before Shinji could respond Ren appeared "Takamizawa! What were you doing in there? I never would've thought to see you run so fast."

The rider laughed, Shinji turned to Ren "Why were those monsters working together? That doesn't make any sense!"

"The same thing has been going on for a while...you have heard of the missing families right? I was at one of their houses fighting the Mirror Monsters when even more showed up...something is bringing them together and it isn't good." Ren answered.

Shinji turned to Itsuro "I'm guessing you are at a loss as well..."

"Actually while invisible I was scoping through all the monsters...I pinpointed a mass herd in a Mirror Factory...one of the big smoke exhausting industries along the coast...if someone is behind this Mirror Monster Union...then that is where they would be located."

Shinji scratched his head "That makes a lot of sense...you did that just to embarrass us right?"

"Embarrass you." Ren and Itsuro answered at the same time before walking away.

"Damn, this is worse than Asakura agreeing to the fact that I am an idiot..."

"It's your fault for believing he was thinking otherwise!" Ren answered back before starting up his cycle and driving off.

Shinji stomped the ground and turned to the now empty house, a new fight has begun and despite the fact that the monsters were taking more people, it seemed to Shinji that the riders might finally work together.

Upon returning to Atori, Shinji was surprised to see several people greatly enjoying meals "That is...odd...today is the quiet day."

He turned to see Obaa-san "Shin-chan, I have to introduce you to another nice young man...but after what happened with several other of my recent workers...don't mess this one up!"

Shinji nodded and saw someone behind the counter preparing a dish with several vegetables lying around "You! You've been here all this time?"

The man turned "Ah...you gave me the 'don't order coffee' tip several hours back...yeah I stood around to prove to Obaa-san, that I can make amazing dishes with just vegetables, she really didn't believe me when I said I made a tomato cake before."

Shinji nodded, the other man seemed overly cheerful and carefree, sometimes Shinji wished to be like that, but with the Rider War taking an unbelieveable turn, there was no time for that "Right well...what is your name?"

The man turned back to him "Oh right, sorry, my name is Tsugami Shouichi."

If you don't know the Ryuki Mirror Monsters by name, wiki is the way to go, that is what I used (trust me, after reading the descriptions, if you've seen Ryuki twice like I have, you'll know what it looks like)

So this story takes place during Ryuki and possibly after the end of Agito or near the end of Agito (which also chronologically places it AFTER Agito V Kuuga)
It's a somewhat alternate reality of Ryuki in which Sudou didn't die in his fight against Ren (the fight happened differently, his deck was never hit) and Itsuro is a part of the war
Hope you enjoy the story as it goes!