Here is some gameplay footage of the fight against Void Termina, the final boss in Kirby Star Allies:
Seems like there were some parts about Void Termina I was right about, and parts I was wrong. He does indeed have a Zero/Dark-Matter like form, but not in the way I expected (I thought he would literally look like the logo that appears on Hyness's robes and the Jambastion Mages hats). The symbol with the heart-shape with horns is actually his mask that you see for the duration of the first fight (as a side note, I hope I'm not the only one who thinks that mask gives Majora's Mask vibes)
I gotta say, this fight looks pretty unique. It has a true 3D feel to it in the movement and navigation aspects as you try to strike Void Termina's weak point. Seems like the first phase is you fighting his "giant" form. The mere setting of the fight, the form the boss takes, and the swords he uses reminds me of the Demise battle from Skyward Sword. Void Termina also takes on a bird-like form for this phase of the fight, and it looks pretty cool from the footage presented: you harrowly dodge his incoming tackle to strike his weak point on his back.
Second phase of the fight appears to be the fight with its "heart" inside its body. The mere shape and patterns on it reminds me of Anankos's soul form from Fire Emblem Fates. What's also worth mentioning about this phase is that if you look closely in the background, you can see Hyness and the three Jambastion Mages wrapped in a cocoon-like substance, like their now deceased bodies are having what's left of them absorbed for Void Termina to use as energy. Creepy stuff!
Not much is shown of the Zero/Dark-Matter form portion of the fight other than the conclusion of it. Seems like the anime reference they're going for with Kirby's final attack this time is is Gate of Babylon from Fate/Stay Night, particularly in the way the allies appear behind Kirby and are thrown as projectiles to finish Void Termina off. In Planet Robobot, Kirby's final attack against Star Dream basically ripped off Guren Lagan. Makes me wonder what anime reference will Kirby utilize for the final boss in the next main series game.