(At the outskirts of Kuoh, a mansion that belongs to Chris Dragonius is seen as it's surrounded by a large garden. Inside the mansion we can see a throne room where Chris was being pleasured by his peerage as he smirked feeling it.)

Chris: This is so nice.

Maya: As it should my little brother~. You deserve all the love~.

Chris: Oh right~. I should get to work on finding a way to make all those girls mine~. But how~?

(Soon the door opens up as Saizel enters the room.)

Chris: Saizel? What brings you to here?

Saizel: I've got some interesting news for you.

Chris: What is it?

Saizel: Turns out we have a guest roaming around and looking for a group known as The Foundation. It's a girl.

(Chris' eyes widen as he smirks.)

Chris: Well then, it seems that we must get going then~.

https://youtu.be/Q0FOg8I5RZI

(Mere days have passed, and new problems have come to light. Basically a triple trouble: Issei, Rias, Akeno and Koneko got captured and eventually under the control of The Foundation as their energies became power sources for four Gundams that said group took possession, Chris Dragonius came into picture as he owned a third of Kuoh, and Kokabiel is planning to start another Great War.)

(Luckily, Sarada had a plan to save the four devils as he gave Y/N's group the coordinates of The Foundation's location in the Underworld as they go for it to rescue them. What they don't know is that Mei is watching them as she creates a magic circle to contact Chris.)

Chris: Yes, Mei?

Mei: Lord Chris, I have managed to locate the group. Shall I continue to follow them until they arrive to their location?

Chris: Yes. As soon as they do, please contact me. And me and the girls will be there soon.

Mei: As you wish, my lord.

(She ends her call, and keeps following the group.)

(Timeskip brought to you by this.)

(Y/N's group is seen heading to the Underworld, and found what looks like a base.)

Y/N: There it is!

Xenovia: What now?

(Y/N pulls out his driver and places it on his waist.)

ZIKU-DRIVER!

(He then pulls out two Ridewatches, and clicks them.)

ZI-O!

D-D-D-DECADE!

(He then places both Ridewatches in his driver, and prepares to transform.)

Y/N: Henshin!

RIDER TIME! KAMEN RIDER ZI-O! ARMOR TIME! KAMEN RIDE! WOW! DECADE! DECADE! DE~CA~DE~!!!

(As Y/N transforms, Woz comes in to rejoice.)

Woz: IWAE! The one to inherit all Rider powers, the time king who will rule over the past and the future. And his name is Kamen Rider Zi-O: Decade Armor!

Zi-O: Ikuze... (determined) TEMERA!

(Zi-O rushes to invade the Foundation Base as they go to rescue the four devils. As they go further into the base, they see Sarada, Feixiao, Jingliu and Acheron fighting a horde of soldiers and Ryu Black himself.)

Zi-O: It's him...

(Sarada is seen clashing with Ryu Black as the latter is seen in his Armored Core.)

Ryu: I'm impressed you could keep up with me.

Sarada: I'm not exactly human, though the feeling is mutual.

(The three girls are seen taking care of the Foundation Soldiers, who try to shoot them down, but they easily strike them down with their powers, and in Acheron's case, her sword.)

Claire: They seem to be handling the situation very well.

Zi-O: While it may be true, they have this Ryu Black, who captured my brother. He's the main priority at the moment.

Irina: So let's help them out.

Xenovia: Fine!

(Then Zi-O summons his weapon.)

RIDE HEISABER!

(He then slashes the soldiers one by one as Ryu Black growls in annoyance.)

Ryu: Tch, persistent...

Sarada: You came, Zi-O.

Ryu: Did your girl locate my brother and the others?

Sarada: (nods) They're in the hangar, though you won't like what they've done to them.

Ryu: Ha! Too little, too late, Zi-O. Issei, Rias, Akeno and Koneko are now the Foundation's properties!

Zi-O: (pulls out a Ridewatch) Damn you!

HEY! KABUTO! DUAL TIME BREAK!

(Zi-O launches a projection of the Kabuto Zecter, and it hits Ryu's mecha, making it stagger back.)

Ryu: Damn it!

(He then pulls out the Ghost Ridewatch, and inserted it at the Decade Ridewatch.)

FINAL FORM TIME: GHO-GHO-GHO-GHOST!

Zi-O: I've got another thing to you!

(Zi-O then summons Nobunaga, Tutankhamun and Houdini into the battle.)

(Three of the parka charge at Ryu and attack as Tutankhamun use Gan Gun Hand in Rod Mode and slash him while Nobunaga fire his shots at Ryu using Gan Gun Hand in Gun Mode.Houdini then jumped and deliver a drill kick at Ryu as his mecha is somewhat damaged.)

Ryu: What are those goddamn things?

Zi-O: These are parkas, and now you will pay for my brother and his friends.

GHO-GHO-GHO-GHOST! FINAL ATTACK TIME BREAK!

(Zi-O summons the Ghosts of the 15 Heroic Eyecons. They all appear behind the Ghost symbol before Zi-O and the 15 Heroic Ghosts performing Rider Kick versions of their Omega Drives to defeat Ryu, whose mecha got badly damaged as it lost one of its arms.)

Ryu: Darn it...

(Suddenly the Foundation Man speaks through the radio.)

Foundation Man: Looks like you need backup, Ryu Black!

Ryu: (growls) Just send them here already, damn it!

Foundation Man: My, my, don't be so impatient.

(Suddenly four Gundams come out as they stand in front of the heroes, giving Ryu time to escape as he glares at Zi-O.)

Ryu: It's not over yet, bastard...

(He flies away as the four Gundams' eyes glow.)

(Zi-O gets shocked about what happened to his brother and the others.)

Zi-O: (eyes widen) Brother...

Kuroka: (eyes widen) Shirone...

Kiba: (growls angrily) You son of a... what have you done to them?

Foundation Man: As you can see, they are now part of my project. I hoped to test them before they could be ready to fight, but since you're here.... Gundams, destroy them!

Issei/Rias/Akeno/Koneko: (unison) As you command...

(The four Gundams prepare to attack as the screen fades out.)