Disclaimer: the world of RWBY is created by the late great Monty oum and the world of kingdom hearts is created by Tetsuya Nomura so in any regard I don't own anything of these franchises.
The warehouse, remnant
Blake: (holding her blade to defend the recovering Ven) stay away from my Ven!
Vanitas: (laughs maniacally at blake's response) Your Ven? oh please! Give me a break... What made you think he would care for an animal like you? As far as I know... Ven would let you die the minute he has a chance to survive... It's who he is and who he'll always be! (Raises his keyblade upward to kill her) Because He... Is... Me!
(Just as he was about to slash down and end her life Ven reawakens and blocked the attack)
Ven: (panting heavily as he hasn't recovered yet struggling to hold his guard) Don't... You... Dare... Hurt... Her!
Blake: (getting worried Ven hasn't fully recovered yet) Ven...
Ven: (panting a bit) I won't... Let him... Hurt you Blake... I promise... I will protect you!
Blake: (starts tearing up) V-Ven
Bgm: enter the darkness
Vanitas: (smirks as he pushes his keyblade Down further) Your still able to fight huh? Well then venny boy... Let's see how strong... you have become!
(Vanitas grabs Ven's arm and throws him across the warehouse and goes underground with his eruption attack stalking Ven like a shark hunting its prey lucky Ven moves with a dodge roll but as vanitas emerged from the ground in a leap to the sky a flurry of firaga orbs damage Ven in the process and vanitas quickly grabbed Ven's leg to throw him on the ground and casted dark thundaga searing parts of Ven's body. Ven gets up and rolls out of the way to use curaga and heals himself and then uses strike raid to hit vanitas only for an afterimage to take the second hit and vanitas appears above Ven)
Vanitas: (from above Ven as he) pathetic!
(As vanitas was going to spike on the ground, Ven moved out of the way and flicks his wayfinder to activate a d-link and bolts towards vanitas ramming him with a blue burst barrier)
Ven: (smirks as his new form tricked vanitas) how's that for pathetic?
Vanitas: (noticing his appearance as he now has white gloves, red shoes with a white stripe and a golden shoe buckles and green eyes and blue hair begins to clap slowly) well... You chose the sonic d-link didn't you?
Ven: (scoffed as the answer) what's it to ya?
Vanitas: the fact is... That I knew this would happen so... (Puts his arms to the side) I just have to show you, the dark side of d-links!
(Vanitas's suit begins to change with the glow of darkness as the red heart on his chest turns to two white v's and his boots turns to hover shoes of black and red with golden hover exhaust pipes on the bottom and gets his black gloves turn to white gloves with golden rings on his wrists and his hair gets some red stripes and his eyes turn red)
Blake: (looks at vanitas in shock) that's impossible! He can't do that can he?
Ven: so...(clutching his fists) you can d-link as well can you?
Vanitas: (looks at his new form and smirks) yep but, These are more dark d-links than the normal ones your use to as these links use the rivals and enemies of the worlds you visit. (Starts walking around and Ven mimics him) and why not start with the ultimate life form: SHADOW THE HEDGEHOG!
(Vanitas skates towards Ven as he runs towards him colliding their keyblades on impact)
Center of vale
(Team STRV minus Ven and team RWBY minus Blake with qrow sees a purple behemoth heartless right in front of them)
Terra: (looking in awe) A behemoth... Here in remnant?!
Roxas: I'm just as shocked as you are (seeing it's about to attack) uh oh... Uh sora?
Sora: (seeing the behemoth ready to strike starts stepping backwards and turns around to the group) EVERYBODY BACK!
(The behemoth roars letting out a shockwave shattering all the glass in a 50 mile radius and blows the teams away 30 feet from the behemoth and sora, Terra and roxas catch Ruby, yang and Weiss respectively while qrow uses his acrobatics to balance himself and lands on their feet)
Qrow: (wipes his face with one hand) you boys deal with that all the damn time?!
Terra: (scratches his head) actually we've face worse than this thing.
Yang: (shakes her head) wait... Worse?
Roxas: (chuckles softly) he's right we did face worse than the behemoth here.
Weiss: l do love to enjoy some memory searching but we got Grimm right behind us!
(The team looks behind them to see 15 Ursa Grimm behind them)
Sora: (points to the behemoth) my team will focus on the behemoth while you guys-
Ruby: (finishing what sora was saying) while we handle the Grimm? Got it! (Sees the behemoth go on its hind legs and slam the ground making spikes head towards the group) SORA LOOK OUT!
Sora: (looks at the incoming attack) I got it!
(Sora jumps back and spins his keyblade like a baton and speed steps forward slashing rapidly left and right until he gets to the other side of the attack near the behemoth and three streaks of light cut the attack into rubble letting Terra and Roxas go on to catch up)
Qrow: (looks in awe) whoa...
Ruby: (sighs lovingly) that's my sora.
Yang: um... guys less talking more Grimm butt kicking!
(The group of Ruby, yang, Weiss and qrow run towards the hoards of Ursa and beowolves to prevent more destruction while sora, Terra and Roxas head towards the behemoth with keyblades in hand)
Terra: (running ahead) sora! What's the plan?
Sora: (looks at the behemoth and gets a plan ready) Terra get to the behemoth's legs and hold it down! (Looks to Roxas) Roxas get on its back and attack it there are no defense on it! I'll distract it and we take it down together!
Roxas: (nods and gets airborne) you can count on me!
Terra: (salutes while running) yes sir!
(Sora gets in front of the behemoth and begins slashing at it, drawing it's attention as it casts raises its paws for another attack, Terra causes quake on the spots the creature was going to land and making sink its front paws in the pavement and did the same for the back legs trapping it. Roxas gets on the behemoth's back and impales it's back with oathkeeper and oblivion and five beams of light with the organization insignia in a pentagon formation damage it more.)
Sora: great work guys, Let's finish this!
(Just as sora was about to deliver the death blow to the behemoth but wasn't paying attention as from its mouth fired a huge orb of darkness impacting Sora and sending him flying)
Roxas: (saw what just happened) SORA, NO!
Terra: (in shock seeing the behemoth's best attack fired in an instant) that's impossible! How did it use the dark burst in an instant?!
(Just as Terra said that the beast jumped out of the quake pit fall and began spinning in a ball very quickly getting Roxas sent flying into a near by building and crushed Terra as he tried to guard the new attack with no luck)
Meanwhile with team RWBY with qrow minus Blake
Yang: (punching out an Ursa dodging left and right) hey sis, how are things on your end?
Ruby: (hooking her scythe to a beowolf's neck and decapitated it) so far so good! How bout you uncle- (sees qrow surrounded
Qrow: (jumps into the air and heel kicked and sliced down 3 Ursa's in record time) what do you think?
Weiss: (looks on as she impaled 3 beowolves with her ice dust tipped Myretenaster and then slashes them to ribbons) he's doing just fine Ruby don't worry! (Looks to see a dark orb flying through the sky heading upwards) what on remnant was that?!
Qrow: (gets a close look to see the orb and someone in front of it) um Ruby... You'd better see this...
Ruby: (rushed over to see her uncle) what's going on?
Qrow: (hands her some binoculars from his jacket) looks like your boyfriend's in trouble.
Ruby: wait... What?! (Looks through the binoculars at the front of the dark orb to see sora struggling to defend himself) *gasps* SORA!
Sora: (taking damage as he's trying to escape) ok... Gotta think fast! (Suddenly an idea popped up) alright then... (Pulls his keyblade arm back) let's hope I can swing for the fences!
(Sora with one arm uses his keyblade and smacked the dark burst orb off of him but it exploded, causing a shockwave to hit him and sending flying towards the ground and impacted hard on the pavement causing a crater leaving him unconscious)
The warehouse
(Ven and vanitas were still at super speed clashing clearly with ven at a disadvantage as vanitas is warping around with chaos control rather than using his speed which is disorienting him)
Ven: (panting heavily) h-how is he this f-fast?
Vanitas: (gets behind ven and strikes his keyblade on Ven's back getting him down to both knees) it's over... Ven.
(Ven spin kicks but missed as vanitas ducked out of the way and grabbed him in a chokehold from behind)
Ven: (gasping for breath) s-shut up! I'm not... Giving up!
Vanitas: (shakes his head) let's finish this... (His body glows a red aura) CHAOS... BLAST!
(An explosion of red energy engulfed both vanitas and Ven leaving only smoke and both men still standing but Ven's eyes are closed and his sonic d-link is gone with his body coverd in scratches and blood coming out of eack cut as Blake looks in horror seeing ven beaten like this)
Blake: (seeing Ven tossed to the ground and looks at vanitas in pure hatred as he is holding Ven's wayfinder) let that go... It doesn't belong to you!
Vanitas: (looks at it) your right... (Drops the wayfinder and heel stomped it breaking all the green glass around it and bending the metal frame upward) but I don't care... What happens next.
Blake: (gets up and runs towards vanitas with her gambol shroud) you bastard!
(As she slashed vanitas, it was only an afterimage and he just slashed Blake only to have get caught by a red dust clone and exploded on impact damaging vanitas only a small bit)
Vanitas: not bad Blake... But let's see if you can dodge this...
(Vanitas dives into the ground and begins to stalk Blake as she is running around but as she tried to turn left, vanitas caught her and slammed her to the ground and just as she got up... Vanitas struck her with dark thundaga paralyzing her)
Blake: I Can't move. (Shifts her eyes up) what did you do?
Vanitas: simple... Dark thundaga to your kind and people of this world get paralyzed on the slightest touch and suffer a bit while keyblade wielders just get damaged. (Sticks the void gear to Blake's neck not impaling her) it's too bad I have to kill you... (Licks his lips) Your pretty hot
Blake: (spits in Vanitas's face) go to hell!
Vanitas: (wipes the spit off) feisty, I like it... But, a wars a war. And besides... In war (raises the void gear upward) there are no prisoners. My kind of rules.
Blake: (Closes her eyes as she began to lose hope once more) i guess hope... is just an illusion.
(As he was going to kill Blake, Ven woke up and using his speed stepped in and blocked the hit in the knick of time but, with his body leaving a large gash on his chest)
Ven: (with one eye closed bleeding from his mouth shaking a bit) I... Told... You... Before... Don't you... Touch her!
Blake: (tears start pouring down her face as she saw ven's blood dripping) V-Ven.
Vanitas: (smirks as he adjusts his keyblade) your persistent aren't you? (Looks down) But answer me this... Why?
Ven: huh?
Vanitas: (shouts out) why are you sacrificing everything for her?! You have only met her for one... Damn... Day! So answer me why?!
Ven: (coughs a bit) because... She gives me the strength... To keep fighting on... To grow more powerful... And also her heart is as gentle as the light and sure... We may have darkness inside us but... Together we can overcome it and also I...
Vanitas: (holding his keyblade upward to kill Ven) you... What?!
(Vanitas swings his blade swings down but Ven blocked it with his wayward wind which begins to glow and transformed into the lost memory keyblade and a blue aura circle appeared around Ven's feet and heals his wound as water begins to rise from it.)
Bgm: ash-greninja theme song
Ven: (his eyes turn red) I...love her... (Screams to the sky) WITH EVERYTHING I'VE GOT!
(The water ring turns into a water tornado as it erupted from the ground making Vanitas step back and Blake smiling with tears in her eyes from not only his confession and as Ven turns into his "ash-greninja" fusion seeing the hopeful teen she fell in love with return to win this fight)
Vanitas: (looks in pure shock as he jumped back) that's impossible?! I destroyed the wayfinder! You can't do this!
(The tornado around Ven bends around to form wings and then morphs into the huge water shuriken which is on his back that he throws at vanitas so quickly it broke the sound Barrier and damaged vanitas and sent him into a crane, destroying it and makes him drop to one knee)
Ven: (smirks) I don't need a wayfinder to show you the power of my bonds because they were made with one purpose... (In the blink of an eye Ven grabbed the broken wayfinder and restores it to full condition which he turns around and puts it on Blake's neck to keep it safe) an unbreakable connection, That's what makes me... (Points to vanitas) stronger than you!
Blake: (clutches Ven's wayfinder close to her heart and begins to blush) Ven I-
Ven: (turns around and puts a finger to her lips) tell me later...(He winked at her) ok? (Puts a hand on her shoulder) Keep it safe for me?
Blake: of course but ven... When you survive... (She blushes at him) you owe me a kiss.
Ven: (gives a thumbs up) you got it Blake... (turns to vanitas holding lost memory) but first things first I gotta win...
Vanitas: (stands up wounded but is smirking) looks like you finally grew up Ventus... Now get ready!
Ven: (smirks as for the first time ever he agrees with vanitas) cause here we go!
To be continued.
Authors note: hey guys! thanks for all the support so far and get ready for the next two parter called "the power of synchro" so get hype and get ready as "ash-greninja" Ven takes on vanitas. also what are sora and the others gonna do about this strange behemoth? Also I know it's strange for Ven to get up so quickly and d-link without his wayfinder but, don't worry all will be explained next chapter so remember.
DFTBA!