It begins where Y/n came face to face with Tsukasa in the middle of the war.

Decade: So, your a Kamen Rider too.

Y/n: That's right, and I've been looking for you, Decade.

Decade: And what are you here for?

Y/n: You should know why I'm here. *puts of Ziku Driver*

Blade A.R.: Good. So even he knows Decade is the enemy.

Y/n then grabbed his ridewatch.

Ridewatch: (Digital beeping) Zi-O!

Y/n: Henshin.

Rider Time! (Electronic guitar) Kamen Rider Zi-O!

Decade: I see, I know why you're here.

Zi-O: That's right.

Y/n then summoned his weapon.

"Zikan Girade!" "Zyu!"

But as soon as Y/n summoned his weapon in blaster form, he immediately turned around and started blasting at the 9 Heisei Riders.

Unfortunately, all of them took a stand back, as Y/n's line of fire missed.

Natsumi: What?

Zi-O: I came to help you.

Kiva A.R.: ...Hey! What are you doing?

Decade: *walks to Y/n*What's the meaning of this?

Zi-O: Unlike them, I'm on your side.

Decade: But why would you help me?

Zi-O: Talked with that old man Eijiro back at that photography studio. He told me about your situation and what you've been doing.

Decade: I see... But the reason why they're trying to defeat me is because I am this Destroyer of Worlds.

Zi-O: You're talking like you accepted something those guys forced you to be. But I heard you've been helping and saving those worlds. If you're this so called "Destroyer of Worlds" then what did you do to be called like that?

Decade: I didn't, at least not yet...

Zi-O: Then why are you doing this?

Decade: Because I was told to save the Rider Worlds from being destroyed. But instead, I was told by him to kill the riders instead.

Y/n then turned to face the A.R. Wataru Kurenai.

Zi-O: So, you're the reason that Decade wound up into all of this.

Kiva A.R.: *takes a few steps forward*Yes I am, and it's the truth that Decade will destroy us all.

Zi-O: But you were the one who told Tsukasa here to save the rider worlds. Why are you forcing him to be someone who's not?

Blade A.R.: You wouldn't understand even if we explained. If you're standing alongside Decade, then we'll eliminate you as well.

Decade: Zi-O, you don't need to do this, this is my fight.

Zi-O: And let you become what they want? No way. Besides, I'm starting to hate these guys trying to kill you after all the the good things you've done. It makes me feel sick of it.

Decade: ...I understand. Then thanks for the assist, Zi-O.

Zi-O: No problem, Decade.

The 9 Heisei Riders then started to charge at both Y/n and Tsukasa.

Y/n and Tsukasa then grabbed their swords and are about to charge as well.

Decade: Let's go!

Zi-O: Right!

Then the big battle has begun, as Y/n and Tsukasa faced the 9 Heisei Riders, while Kaito and Natsumi are watching behind the scenes of the event.

Y/n and Tsukasa faced the 9 riders one by one randomly, while working together for team attacks.

After a little while, from Tsukasa, shows Y/n in the middle of fighting until he clashes swords with Shinji.

Ryuki A.R.: Why? Why would you side with Decade!? I thought you were fighting for good when you saved me.

Zi-O: That voice...

Y/n then had a flashback of him fighting Ryuga back in the Mirror World.

Zi-O: I see, you were that guy I saved back in 2002.

Ryuki A.R.: ...Stop! Do you have any idea what you're doing!? Decade needs to be stopped so this fighting will end.

Zi-O: That's true, but Decade is also a Kamen Rider, fighting to protect and save others.

Y/n then grabs the Ryuga Ridewatch.

Zi-O: I may have saved you last time, but don't assume it will happen again.

Ridewatch: (Digital beeping) Ryuga!

Y/n then puts it on the other side and spins it around.

Armor Time! (Dark electronic beat mixed with V-Buckle-esque music) Advent! Ryuga!

The armored parts appeared in front of Y/n in physical form, doing the pose of readying itself before performing the Final Vent attack. Then it was put onto Y/n, as well as the kanji.

Everyone present were shocked upon Y/n's transformation.

Diend: He transformed to another rider?

Natusmi: He's just like Tsukasa...

Decade: I never thought anyone would have a smiliar ability to mine.

Ryuki A.R.: That... You look just like the other me.

Zi-O (Ryuga Armor): I abosrobed his power to save you. Now I'm going to use this power, to defeat you. Hyah!

Y/n faced some of the riders, as well as Shinji, hand to hand for a bit.

Suddenly, Y/n's two Black Drag Visor Zs on his shoulders have their eyes blinking.

Black Drag Visor Z: Strike Vent! X2

Then, the two Black Drag Visor Zs got out of Y/n's shoulders and were equipped to Y/n's hands.

Y/n then went back to fighting hand to hand. He even used the Black Drag Visor Z on his left hand to shoot black fireballs repeatedly.

Shinji tried to get in close, but Y/n held out his right Black Drag Visor Z and black fire was shooting out from the mouth like a flamethrower, critically damaging Shinji.

Y/n's both Black Drag Visor Zs then had their eyes glowing again.

Black Drag Visor Z: Advent! X2

Both gauntlets then both got attached to his arms, leaving his hands free.

Suddenly, Dragblacker descended from the sky and appeared alongside Y/n.

Zi-O (Ryuga Armor): Get them.

Y/n then ordered Dragblacker to attack all the riders.

Dragblacker did some twists in turns in tackling the riders, bashing them with his tail, and breathing black fire everywhere.

Y/n then helped out dealing extra damage by engaging the riders, attacking them with his black fiery fists.

Decade: Huh, not too bad. I better do the same as well.

Decade Driver: AttackRide: Advent!

Tsukasa summoned Dragreder thanks to his Rider Card.

For a little bit, both dragons worked together in dealing with the riders.

Then Y/n and Tsukasa are side by side with each other.

Zi-O (Ryuga Armor): You thinking what I'm thinking?

Decade: Way ahead of you.

Y/n pressed both ridewatches and turned his driver, while Tsukasa inserted another Rider Card.

Finish Time! Ryuga! Final Time Break!

Decade - Final AttackRide: R-R-Ryuki!

As both Y/n and Tsukasa jumped and flipped high in the air, both dragons swirled around.

Then both performed their rider kicks when the dragons breathed fire on them. While Tsukasa's attack is normal to the original Ryuki's, Y/n's attack shows a picture of a dragon while being coated in black flame.

Both Y/n and Tsukasa were able to take out Ryuki, Agito, and Faiz.

Y/n transformed back and he and Tsukasa stared at the remaining riders.

Blade A.R.: I didn't think this would be easy anyway.

Zi-O: Then this will be difficult once you get a load of this.

Y/n then pulls out the Glaive Ridewatch.

Daiki: That face. Is that...?

Ridewatch: (Digital beeping) Glaive!

Y/n then repeated the same process.

Armor Time! (Glaive Buckle standby noise) Open Up! Glaive!

Like before, the armored parts appeared physically and did its form before it and the kanji were installed onto Y/n.

Daiki: ...! He looks just like Junichi.

Blade A.R.: What is that form?

Zi-O (Glaive Armor): I wouldn't be surprised if you don't know this guy who I stole his power from.

Y/n then engages again.

Decade: If he's going with that form, then I think this will be more fitting.

Tsukasa then pulled out the Blade Ride card and puts it in.

Decade Driver: Kamen Ride: Blade!

As both riders are fighting the enemy riders, Y/n is fighting using the super abilities from the cards.

Everytime Y/n uses an ability or a combination of them, one of the Rouze Cards on his shoulders lits up.

On Tsukasa's side, his side of enemy riders attempted to attack altogether, but he places another card to his driver.

Decade Driver: Attack Ride: Metal!

Tsukasa's body turns to metal which bounces back the riders attacks.

Tsukasa then puts another card in his driver.

Decade Driver: Attack Ride: Mach!

Tsukasa then attacked his enemies with lightning speed with his sword.

Zi-O (Glaive Armor): Time to finish this.

Decade (Blade): Finish time already, huh?

The two riders then repeated their own processes in performing thier finishing attacks.

Finish Time! Glaive! Gravity Time Break!

Decade - Final AttackRide: B-B-Blade!

For this version of their rider kicks, Y/n's Rouze Cards were picked off and absorbed to his right leg, performing a rider kick version of Glaive's Gravity Slash.

As for Decade, he performs the original Blade's Lightning Sonic.

The both were able to take out three more riders, Hibiki, Kabuto, and Den-O.

After both Y/n and Decade tranformed back to their normal forms, all that was left are Kiva, Blade, and Kuuga.

Decade: Hey Zi-O, can you copy this?

Decade Driver: Kamen Ride: Kiva!

Zi-O: Eh. Sort of.

Y/n then grabbed the Dark Kiva Ridewatch.

Kiva A.R.: Wait, that's...!

Ridewatch: (Digital beeping) Dark Kiva!

Y/n then put his ridewatch on the other side and repeated the process one more time.

Armor Time! Wake Up! Dark Kiva!

The armored parts appeared in a pose of letting its hand being bit by a giant version of Kivat-bat the 2nd before being put onto Y/n. And the giant version of Kivat-bat the 2nd split into two at attached to the shoulders.

Decade: I'm beginning to like the way you think, Zi-O.

Zi-O: Heh, thanks I guess.

With that, Y/n and Tsukasa went to finish off the remaining riders.

It was an intense battle with the tree riders left over.

As both sides were exhasuted after a lengthy fight, Y/n and Tsukasa decided to end it.

Finish Time! Dark Kiva! Darkness Time Break!

Decade - Final AttackRide: K-K-Kiva!

Tsukasa performs his rider kick similar to the original Kiva's Darkness Moon Break.

Y/n readies his finale with both sides of Kivat-bat the 2nd being put as a whole again, and bit Y/n's right leg. Kivat-bat the 2nd helps carry Y/n up in the air while hanging upside down.

With that Y/n and Tsukasa performed their rider kicks.

However, the two only targeted both Wataru and Kazuma.

The only one left is Yuusuke, who is still in a fit of rage.

Decade: Yuusuke... Let's just end it.

Zi-O: Agreed.

One last time both Y/n and Zi-O repeated their processes one last time.

Finish Time! Time Break!

Decade - Final AttackRide: D-D-Decade!

After the two riders' signature rider kicks, Yuusuke de-henshins, unconscious once again.

Y/n and Tsukasa also de-henshin too.

Tsukasa: Yuusuke!

Tsukasa, Natumi, Daiki, and Y/n went to check up on Yuusuke.

Natumi: Yuusuke...

Tsukasa: Natsumi, how is he?

Y/n: Wait, he's not your enemy?

Daiki: This kid has been traveling with Tsukasa for a while.

Y/n: I see.

Slowly, Yuusuke was seen with his eyes open.

Tsukasa: ...!

Natsumi: Yuusuke!

Yuusuke: Agh *slowly gets up*. Tsukasa. Natumi. Kaito. What... happened to me?

Natsumi: Don't worry about it, everything's alright.

Then they all laughed together.

But then, Tsukasa got up and faced at the defeated 9 Heisei Riders, whom all got de-henshined.

Kazuma A.R.: Decade... You were supposed to defeat us.

Tsukasa: And I have.

Wataru A.R.: But you purposely kept us alive. Why...? After everything that's happen.

Y/n: Because this guy here, never gave up. Even when he's up against a wall and the whole world, or worlds are against him, he still have people by his side who still believe in him.

Tsukasa: ...Wataru, right? Kazuma... and the other riders. If you truly care about saving the rider worlds, then please stop focusing on the solution of ridding Decade out of existence.

Wataru A.R.: ...

Kazuma A.R.: Whatever happens next will be because of you. By continuing down this path, you will truly be fated in destroying the rider worlds.

Tsukasa: Then I will fight against that fate, and will continue to find a way to save the worlds. That was my intention since the start of my journey.

Y/n: You even had the understanding of fighting against fate, or did you forgot it?

Kazuma A.R.: But that's-!

???: Curse you, Decade!

A man's voice has screamed from afar, getting the attention of everyone.

It was Narutaki, the man who mysteriously tries to destroy Tsukasa throughout his journey.

Tsukasa: Narutaki...

Narutaki: Not only you received help from a fellow dark rider, but you refuse to abide to the cycle of creations comes after destruction! In that case, I will have to do it myself.

Narutaki then transforms himself into a random monster and came down to face the challenge.

Narutaki (monster): Now Decade, it's time for you to be destroyed by my hands! I will be the one to make sure your journey will be over.

Y/n then suddenly came up in front of Tsukasa.

Tsukasa: Y/n?

Y/n: Tsukasa said it himself, he chose the path to save the worlds from destruction. I won't let you kill him after he made his resolve.

Narutaki (monster): This has nothing to do with you, yet you dare to interfere. Why would you even help a rider like Decade?

Y/n: I may not know what happened in his journey, but I can tell that as a Kamen Rider, Tsukasa did many good things and already saved many worlds. However, people like you who misunderstand other like him, not knowing who they really are and keep getting in their way, are what ticks me off.

Narutaki (monster): Decade is the Destroyer of Worlds, that's why he must be destroyed.

Y/n: Heh, you remind me of someone who's also trying to get in my way. Trying to destroy me too after believing I'm evil. But I aim to be a king to save my world. Even if Tsukasa is like you said, I won't turn a blind eye on someone who's doing the right thing. Because if it were to happen the other way, then what's the point of being a Kamen Rider!?

Tsukasa: Y/n, you...

Narutaki: If that's how you want it to be, so be it. I will simply get rid of anything in my way of destroying Decade.

Y/n: Then bring it on.

Y/n then grabs his ridewatch and the new ridewatch.

Y/n: Now then, bow down to your king!

[There should be a GIF or video here. Update the app now to see it.]

Ridewatch: (Digital beeping) Zi-O! (Digital beeping) Zi-OTrinity!

Y/n then puts his ridewatch first, then after he put the Trinity Ridewatch, it began to open the moment it was put on.

Then, Y/n turned the knob two more times.

Ridewatch: Zi-O! ...Zi-O II! Zi-O Trinity!

As Y/n readies to turn his driver, projections of his Zi-O form and Zi-O II form appeared in front of him on both sides.

Y/n: Henshin!

Trinity Time! Three Powers! Kamen Rider Zi-O! Zi-O II! Zi-O Trinity! Trini~ty!

The transformation process went like this:

First: Y/n transform to his normal Zi-O form.

Second: Y/n's Zi-O II form transformed into 2 wristwatches with each side only having half of its face.

Third: The other Zi-O form laying in front of Y/n only has its face being implanted on his chest.

Fourth: The kanji from the three parts merged together and merged to Y/n's face.

Again, after the transformation, everyone was surprised to see Y/n's final form.

Narutaki: That's his new form...?

Zi-O Trinity: ...I believe your time ...is up.

To be continued...