Actually, looking back at the original series, there is a source for this.
That is the Senjutsu: Lava Release Rasenshuriken, which Naruto used with the help of the Four-Tails' Lava Release Chakra.
On the basis of the Rasenshuriken, Naruto added Lava Release, a fusion of Senjutsu, Fire Attribute, and Earth Attribute. Such a technique possesses the power to burn, cut, and explode.
This is no longer in the realm of forbidden jutsu; even Senjutsu isn't this exaggerated. This is straight-up cheating.
It holds no reference value at all.
Theoretically speaking, the Rasengan can be developed with all attributes.
Lightning, Fire, Earth, Water—they all work.
Ryou chose Fire.
What can Fire do?
Fire can burn, release high temperatures, and explode.
Ryou focused on the explosive property. First, he had to inject Fire Attribute Chakra into the Rasengan, then endure the high temperatures within it while controlling that terrifying explosive force.
Finally, he had to find a way to throw it.
This is an explosive-type Rasengan. It shares the same nature as the Wind Release Rasengan in that it would affect the user, so it must be thrown.
This requires even more powerful Chakra control.
Ryou slowly attempted it according to the experience of his Spore Clones.
A blue-white spiraling sphere of light condensed in Ryou's hand.
This was the normal Rasengan.
Then, Ryou injected his Fire Attribute Chakra into it.
The blue-white color inside the Rasengan turned fiery red, becoming like a fireball spinning at high speed.
It radiated terrifying heat and an amazing explosive force.
The Spore Clones had already developed a prototype; all that was left was for Ryou to slowly perfect it.
The entire room became scorching hot. Obito, sweating against the wall, looked at the Fire Attribute Rasengan in Ryou's hand in horror.
"Ryou, what are you doing?"
Tobi was also a bit terrified: "Ryou-sama, take it easy! If that technique in your hand blows up, the entire the Akatsuki base will be sent flying into the sky!"
As the saying goes: fine wine in a luminous cup, the Akatsuki base flies into the sky.
If Ryou lost control of this, not only him, but everyone in the Akatsuki would be buried with him.
Ryou said disdainfully, "What are you afraid of? Isn't it just ascending to heaven? What's the big deal!"
"Ryou, Ryou, don't talk, it's about to blow!"
"My Lord, my Lord, don't get distracted, I don't want to die yet."
A voice suddenly sounded outside the door.
"Lady Konan!"
"Daibutsu, I've come to find Grandpa Ryou to practice Medical Ninjutsu."
"I see!"
Because Ryou had saved Daibutsu's life, he was now like a second father to Daibutsu. Daibutsu couldn't find any other way to repay him.
So he watched the door for Ryou, acting as a guard.
Ryou quickly withdrew the Fire Rasengan, then punched Obito, Tobi, and Black Zetsu, those three fellows who were neither-human-nor-ghost, deep into the ground.
He did it to stop them from ruining his good time.
"Damn Ryou! This guy is definitely going to deceive a pure girl, we have to stop him!"
"Stop your face! Obito, don't disturb Ryou-sama."
Daibutsu reported to Ryou, and Ryou immediately used the Transformation Jutsu to turn into an old man before letting Konan in.
When Konan entered, she noticed a scorching heat in Ryou's room, as if a thick fire had been burning.
She was somewhat puzzled.
Ryou explained, "The Land of Rain is always rainy and the cave is a bit damp. I just released a little flame to drive it away."
Konan nodded. She was holding a bowl of porridge in her hand; she was quite thoughtful, considering that Ryou might have trouble chewing, so she specially made porridge.
"Grandpa Ryou, I don't have anything better to entertain you with, so please have some porridge!"
Ryou took the porridge from Konan's hand: "Konan, you are truly so virtuous. If I were still my former self, I would definitely marry you."
Konan smiled.
After Ryou finished the porridge, he had Konan sit on a chair.
"Konan, the training for Medical Ninjutsu is very rigorous. Are you mentally prepared?"
Konan nodded firmly: "Grandpa Ryou, don't worry! For the sake of the Akatsuki, I will overcome any difficulty."
Ryou nodded: "Good, first take off your clothes!"
"!!(ﾟДﾟ≡ﾟдﾟ)!? Ah?!"
Konan looked at Ryou in shock. She knew the hardships of practicing Medical Ninjutsu, but it wasn't supposed to be this kind of hardship, was it?
"What 'ah'? This is how we Medical Ninja train. Otherwise, why would there be so few Medical Ninja?"
Ryou coughed violently, coughing until blood even sprayed out.
Seeing this, Konan quickly stepped forward to clean the blood from the corners of Ryou's mouth.
"Grandpa Ryou, stop teaching, okay? What you need most right now is rest."
Ryou said weakly, "Konan, stop talking nonsense. I'm almost at my end. Quickly do as I say, and I will pass all my lifelong Medical Ninjutsu to you."
"This... okay, fine!"
Seeing Ryou looking like he was about to die, Konan figured he really was at his limit. Grandpa Ryou had such a sincere heart; how could she doubt his intentions?
Besides, what was there to be shy about when it came to Ninjutsu?
...Black Market.
Yahiko and Nagato went to town, holding their noses as they passed through a secret passage in a public toilet to reach the Land of Rain's black market.
They found the person in charge of the black market, wanting to buy a batch of Ninja combat supplies.
Kunai, shuriken, Exploding Tags, and the like.
The black market manager had people pack them for Nagato and Yahiko.
Yahiko paid the money with a heavy heart. You could get anything in the black market, but the price was deathly expensive, and there was always the risk of buying fakes.
A while ago, Yahiko and the others had bought a batch of fake Exploding Tags.
At the time, they cursed: what kind of bastard would even counterfeit Exploding Tags?
As they were leaving, Yahiko suddenly stopped and asked the black market manager, "Do you know a person named Ryou?"
Nagato looked at Yahiko somewhat reproachfully. Ryou had treated the Akatsuki with his whole heart and soul, yet Yahiko still suspected him of having ulterior motives. To be honest, it was a bit unfair.
Yahiko explained to Nagato, "Calm down, Nagato. I know Grandpa Ryou is a great philanthropist, but I also want to know his background."
"After all, such high-level Medical Ninjutsu couldn't have been learned while wandering."
Nagato fell silent.
Yahiko had a point. The three of them were where they were today because of Jiraiya's teachings. A secret technique like Ryou's, which could practically bring the dead back to life, couldn't have been learned through wandering.
The black market manager wondered, "Ryou?! You mean Ryou, right? Speaking of him, it's really a pity!"
"Ryou?!"
Nagato and Yahiko looked at each other, both seeing the deep sense of 'this is bad' in each other's eyes. The Ryou at home really was that beast Master Jiraiya spoke of!
"Quick, tell us what happened with this Ryou?"
"Alright, alright, don't panic, let me tell you slowly!"
"This Ryou was a Ninja of the Uchiha Clan. He developed a forbidden jutsu—"