Bleach belongs to Tite Kubo and his corporate overlords. The appearance of Yachiru's sword comes from another fanfic.
Just the very last scene of the Bleach Anime, inspired by a plot bunny on TV Tropes…
We see a long ridge of black, ragged rock, topped by a long row of human silhouettes, under a red and burning sky. The view pans across, and we see a line of Shinigami. The line is anchored on the left by several members of the Vizard gang. We see several captains, including Rukia, Kenpachi and Yachiru, Komamura, and an uncharacteristically grim-looking Shunsui. Several vice-captains including Renji, Ikaku and Iziru are also present. Urahara, Yoruichi by his side, is wearing a captain's cloak as of yore, although he still has that green-striped hat on. In mid-line there is Ichigo's Karakura gang. There is Ichigo himself, his face covered by a Hollow mask, and Ishida, wearing his most pimpin' archer outfit yet and yielding an equally over-the-top giant bow. There is Ichigo's father, in Shinigami outfit. Chad – or, what we think is Chad, since weird striped armor now covers him head to foot. Tatsuki, also wearing a Shinigami outfit and wielding an odd weapon. [1] The line is anchored on the right by three Arrancar, an adult Neliel Tu and a pair of males of vaguely familiar appearance.
Our viewpoint sweeps away and down the ridge, across a valley of black stones, the ground split here and there with jagged cracks from which a sullen red glow emerges. It sweeps to the slopes of the opposite ridge, which is crowded with an army of monsters. Not Hollows (they have no masks), but nonetheless huge, horrible creatures, in an number of shapes and forms, but all glowing-eyed, fanged and clawed and horned, wearing grotesque and spiny armor, wielding gruesome-looking weapons. Some have multiple arms and some have tails and some have great leathery wings. Larger and even more frightful ones top the ridge, and we focus on a particularly nasty-looking giant whose horns burn with a sickly blue fire and who bears a massive, glowing sword. We close in on something small around its' neck, and see what appeared to be a charm is in fact a dead or unconscious Yamamoto in a loincloth, crucified upside down on a black cross, his long beard somewhat comically obscuring his face.
We switch back to Ichigo and his friends.
"Rescue Rukia, rescue Orihime, rescue Yamamoto…this is beginning to turn into a running gag" remarks Ichigo.
"Well," grumbles Tatsuki, idly spinning her peculiar weapon's long "tail" around her "if it's 'Rescue Kon' next time, I'm staying home."
"Hey!" someone unseen protests.
We switch back to the giant on the opposite ridge. It raises its glowing sword to the heavens. The sword bursts into flames, which leap to the red-glowing firmament and sets off an explosion of fire across the sky. It throws back its head and howls. The army of fiends howls back and shakes their weapons. Fires leap from the cracks in the valley floor.
The comparatively diminutive Soul Society army stands stolidly on the opposite ridge. Fiery sparks begin to drizzle down.
Ichigo turns to Ishida.
"We're screwed, aren't we?"
Ishida pauses as he pushes up him glasses, and replies. "Probably."
A harsh bark of laughter rings out. Ichigo and his friends turn to see Kenpachi and Yachiru.
"Wimping out on us, runt?"
Ichigo glares at him. The killing intent proves as effective as chucking spitballs into an active volcano.
"Well, I'm sure this doesn't bother you, Zaraki" Ichigo snarls. "Win or lose, it's going to be a hell of a fight, and that's all that matters to you, isn't it? Everyone on Earth and Soul Society could die, horribly, and you wouldn't give a damn, as long as you were guaranteed more challenges!"
Kenpachi stares at him blankly, and then snorts.
"Man, you're getting boring as you get older. This is the way it is with humans, right? You get more tedious as you age? I really should kill you before you become a waste of my time."
Before Ichigo can respond, a laugh like the ringing of elfin bells (Diskworld elves, probably) peals out from Kenpachi shoulder height.
"Silly Ichii! Of course Ken-chan will enjoy the fight! And of course we're going to win!" Yachiru giggles.
Tatsuki smiles, a bit sadly. "Of course, Yachiru-san. We'll certainly win…"
Yachiru frowns cutely. "You're not taking me seriously, Rezu-chan!"
"Just STOP calling me that, you little…" [2]
"Ken-chan! Let's show them what we've been practicing!"
Kenpachi looks annoyed. "Not now. I want to have some fun first. And I told you not to say anything."
Yachiru makes a small fist and punches him on the side of the head so hard that all his bells ring, although he otherwise seems unaffected.
"Show them now, Ken-chan! They'll like it! Show them show them show them SHOW THEM SHOW THEM…"
"ALRIGHT! Sheez, what a pain..." Zaraki pulls out his sword, (which the keen-eyed will note is no longer notched like a saw) and holds it pointing in the direction of the horde of monsters. A now smiling Yachiru pulls out a teeny little sword with a big daisy-shaped hand guard and points it skywards.
"It couldn't be…" mutters Yoruichi. She turns to Urahara, who grins and winks.
"Oh crap" mutters Ichigo.
We see Shinigami and allies, faces filled with astonishment, surrounded by blinding light. We see the face of the demon chief enveloped in light, his lower jaw handing down to around his chest. We rise towards the red sky as the light builds even brighter, and two voices cry as one with a volume than rings across the width of Hell…
"BAN-KAI!!!!"
THE END
Closing theme
[1] Google images "kubo tite tatsuki shinigami"
[2] Yachiru has heard through the Soul Society gossip grapeline about Orihime's "lesbian stalker", but has never met Chizuru. After seeing Tatsuki and Orihime together, misinterpretation lodged immovably.