Name: Rasutobaretto |The Last Bullet
Number: Hadou # 98
Incantation: With the certainty of the heavens, muster your last breath, use it, call upon Mercury's swift wings, and Mars' strong arms, fire, and sever the thread of destiny with a single bullet, Hadou #98, Rasutobaretto!
Effect: Rasutobaretto is a forbidden kidou, it's existence is only known to certain old record keepers and the people who actually know it. The reason for this is that it is a devastatingly powerful Kidou, with the ability to instantly kill a captain if performed by someone who is extremely skilled in Kidou.
The Kidou requires an extremely large amount of energy to cast, which must be charged, while keeping your hand in a certain position. Folding in the pinky and ring finger, and holding the thumb flush against the side of the hand with the tip cocked up, someone would make the shape of a gun with them, and begin charging hyper pressurized kidou energy along the two fingers, and directly infront of them, similar to Shunko in a way.
This would have to be maintained for a long time. If someone was highly proficient at Kidou, such as the Kidou Corps captain or the Sotaicho, and possibly some of the more Kidou-centric captains, they'd have to charge for around 7 to 8 posts. Most captains would need to charge for ten posts before able to fire this Kidou off, and if you're not captain class? You will die once you fire this. As it is, a high level captain could only fire two of these off before passing out, The Kidou Corps Captain and the Sotaicho could probably manage three, But even after two both of these people would be feeling extremely strained.
While charging, any other Kidou would be impossible for someone who did not specialize in Kidou, or was an extremely powerful captain period. Even then, higher level kidou (60+) Would be absolutely impossible, and anything above 20 would be weakened severely. Shunpo would be slower than usual because of the massive exertion upon the person Reiryoku, and Zanpakuto abilities could even suffer if they depend enough on a person's reiryoku.
Why would someone put themselves through this? It is an extremely powerful Kidou. If launched by regular captain, it would instantly kill any sub-captain class creature it hit. If fired by a race leader? Anything below a high level captain would be one shotted. If by some magic of defense or pure reiryoku power you are not killed, anyone will be extremely wounded. A Captain firing this at a Sotaicho could blow off his entire lower body. There's a reason this Kidou is forbidden and not thought to anyone but the most trusted and loyal captains of the Gotei Thirteen.
When fired, The energy swirls up around the hand and lower arm, before the "gun" is fired, by pulling finger up sharply, and dropping the thumb. At this point, the Kidou would burst forth, the swirls spiralling in on themselves and pulling closer, until the tail of the bullet is only about eight inches long and maybe four wide. The bullet itself would only be the size of a shotgun shell, perhaps as long as two, and moving at a speed comparable to shunpo. If it misses, it will orient itself towards the reiatsu source it was launched towards. If not launched towards a reiatsu source, it will be fired dumbly.
Upon impact with the target, it will fuse into their body, before merging with their reiatsu, and then using it against them, causing an extremely vicious chain reaction that completely destroys the target. If someone is killed this way, it's similar to being killed by a quincy arrow. Their soul is forever gone. This will look like them being consumed from within, their body turning to pure reiatsu and crumbling apart, fading from existence. It'll only take a few seconds, just long enough for them to realize what's happening.
If however, it hits something inanimate, it would explode in a brilliant display of power, with power equal to several nuclear bombs as all the reiatsu used to form it is violently released. However, this would only be physical force and reiatsu bursting out of it. No radiation would be spontaneously formed.