I don't own Trinity Seven or Kamen Rider and they belong to their respected owners.

Arata saw the belt and something were telling him to place it on his waist but before he did his grimoire called him out.

"Don't!" Sora shouted.

"Huh, why?" Arata asked.

"It will be too dangerous for you to have it on," Sora said.

"Do you know what it is?" Arata asked her.

Sora hesitated before nodding, "Yes, it can you power but you'll be risking your life if you have that on you."

"Does it have the power to save the girls?"

"...Yes."

"Then it shouldn't be a problem," Arata says as he placed the belt on his waist and it formed a belt around his waist. "Ok now what?"

"You place the ring on the left side of the belt," Sora instructed.

"What ring?" Arata looks at the ground to see a ring with a lion's face on it. "Oh, there it is." He grabs and places on his left hand. He then placed the ring on the hole of the side of the belt.

SET OPEN!

Arata felt his body freeze as everything turned black and saw a giant metallic black and gold lion. It had is covered in body armor modeled after the four animals that Beast uses in his Mantles: a red buffalo on his breastplate, an orange Falcon on his right shoulder guard and wing, a purple Dolphin on his left shoulder guard and wing, and a green Chameleon on his tail.

"What the hell are you?!" Arata shouted.

"I am Chimera," the beast answered. "Kasuga Arata, you have opened the doors of the belt and become one with me." He explained.

"Door? One with you? I don't understand?"

"In exchange for granting you my power you must feed on my magical energy," Chimera answered. "As of now, I am absorbing a bit of your grimoire's power."

"What?! Don't do that!" Arata yelled at the beast.

"Then you should feed me with the Phantoms magical energy," Chimera said. "Because if you don't your life will run out."

Arata's eyes bug out from his head. Now he knows why Sora didn't want him to use but now he doesn't have a choice.

"Fine, I'll feed you if you help me save my friends," Arata said.

"Good now become my Mage Beast!" Chimera then jumped into Arata which caused a big light blinding him.

~/~

"I can hardly move," Lilith groans in pain.

Right now she and the rest of the Trinity Seven are in after the Manticore poisons them with his tail. They don't know where that Phantom came from but they needed to destroy it. But they didn't expect it to be smart and strong.

"In the next few minutes the poison will kill you," the Manticore smiled. "But I do not like to see girls in pain… so how about I let the Ghouls end your pain."

"Why you!" Lilith tried to get up but fell back to her knees.

A Ghoul walks towards Lilith ready to stab her with one of its spears.

"Lilith-sensei!" Levi cried out.

Before the Ghoul can commit the finishing blow it was kicked away by Arata.

"Arata!" Lilith cried out.

"Husband," Arin said in a monotone voice.

"Don't worry I got this," Arata smiled.

He pulls out his Driver On Ring on his right hand and places his hand on top of his belt.

DRIVER ON!

"Eh?" Akio was surprised to see the belt. "Isn't that belt supposed to be locked up."

"Sorry but master found and took the power," Sora said arriving towards their side.

"That idiot does he know how dangerous that magic is!" Mira said.

"He took the risk," Sora says.

Nobody likes that belt. It's too dangerous with that monster eating all its past users for not fulfilling their end of the bargain.

Arata took the Beast Ring, which is a yellow lion-like face with large green eyes, and placed it on his left middle finger. He then began to spin his arms in a circle.

"Heeeeeeenshin!" Arata says as he inserts his ring into a hole on the side of his belt.

SET! OPEN!

A magic circle appeared in front of him as it moved towards him.

L-I-O-N! LION!

As the magic seals disappeared behind him, everyone was able to get a good look at him and were stunned with what they were seeing. This was actually the first time they saw the armor as they only saw it in paintings.

Arata's new look was him in armor which was also mostly black, but also about a third of it is covered in golden colored pieces, including a pauldron on his left shoulder shaped like a lion's head, his helmet carrying the lion motif still, with large green compound eyes. Arata became Kamen Rider Beast.

"What!" Manticore was shocked by Arata's new power.

"You like? The Chimera called me his Mage Beast," Arata says. "But I feel like more of a Kamen Rider. Kamen Rider Beast, now I like the sound of that." He says before clapping his hand. "Now let's begin the feast!"

Arata charged forward and began punching the Ghouls which were pretty overwhelmed by Beast's power. Beast place his hand on his belt which made him summon his rapier-like weapon, the Dice Saber. Arata begins slicing the Ghouls destroying them one by one as a magical seal appeared on the destroyed Ghoul and sucked in their magical energies and were absorbed into the Beast Driver.

"So this is the power of Beast," Lieselotte says as she looks at the battle.

"Onii-san looks really cool," praised Yui.

"That's my husband," Arin cheered in her monotone voice.

After defeating the grunts Arate turns to the Phantom, "It's your turn but first..." He removed his Driver On Ring with the Dolphi Ring then inserted it on the right of the belt.

DOPHI, GO: DO-DO-DO-DO-DOLPHI!

A golden seal passed over his right arm as he held it out to the side, and placing a purple Dohphin head-shaped pauldron along with a purple and gold mantle on him. He activates the Dohpin Mantle's magic and a magical seal appeared making him throw sparkles of water to the girls.

As the magic hit them, they felt different. Actually they felt better than they were a second ago. They realize that Arata heals them with the Dolphi Mantle.

"The poison's gone," Lilith said.

"A healing spell isn't great," Arata smiled under his helmet.

"What?! You healed them from my venom!" The Manticore shouted.

"Cool isn't it," Arata smirked but took out the Falco Ring. "I'll try this as my finisher." Beast began to switch his Dolphi Ring with the Falco Ring. He then inserts it into the right side of the belt.

FALCO, GO: FA-FA-FA-FA-FALCO!

A magical seal passes through Arata's right arm again as it replaces it with a red and gold Falco Mantle.

Beast jumps up and starts slashing at the Manticore Phantom. He gave it a couple of good kicks before he japed the monster's stomach with his saber which pushed the monster back.

Arata spins the front part of the Dice Saber, it will spin the dice built into the Saber. After that, Beast inserts one of his Falco Ring into the Dice Saber's back part to stop the dice. It stops at number six.

SIX!

FALCO SABER STRIKE!

Arata creates a magical seal in front of him as his blade begins to glow. He then slashes at the seal creating six smaller seals in the big seal with one golden falcon coming from each of the six seals. The falcons flew towards Madicore as they shred through the Phantom. The Manticore grunts in pain as he was finally destroyed as his magical energy flew into a golden seal as the seal was absorbed into the Beast Driver feeding Chimera.

Arata claps his hand together and says, "Thanks for the meal!"

"I always worried that pervert would become a danger if he becomes a Demon Lord but with Chimera inside him, I don't know what kind of path he will go to," Mira said.

Lilith looked at the golden Rider with a worried look, "Arata," she muttered.

AN: Do you guys like it? I watch a quater of this anime two years ago and rewatch some clips a few months ago and I thought this guy could be Beast. I had this idea for a year but never got around to write the one-shot till now. Give me more Rider and anime crossovers.