Kaio-ken (界かい王おう拳けん, Kaiōken, lit. "World King Fist" or "Fist of the King of the Worlds") is a technique invented by King Kai; however, Goku is the only person ever to use it successfully. It is a technique that multiplies the user's Ki for a "heartbeat" - thus increasing their power and speed and enabling them to inflict serious damage to opponents who are considerably stronger than them. However, the downside of the Kaio-ken is that it takes a heavy toll on the user's body, making them more vulnerable to enemy attacks. While in the state, the user's Aura color becomes crimson. The technique requires the use of ki control to handle it properly.
When activating the Kaio-ken, the user gains a red and fiery aura, and in turn, their whole body and clothes acquire a reddish tint. The power of the aura also bristles the user's hair and clothes if they have any. Unlike other transformations (like the Super Saiyan or Golden Frieza), the Kaio-ken must remain with its aura active all the time. There is no change to the user's physique, but the enormous stress of the technique can result in increased blood pressure, popping out veins and swelling the user's muscle mass. The technique can also generate large amounts of heat from the user's body whilst being activated, creating steam rising from their body.
As a result of the Kaio-ken, the user's base Power Level, Strength, Speed, and even Sensory Awareness greatly increase for an instant. With training, the length of activation can be increased, as shown during Goku's extended use of the technique against Frieza. This change is quantified as doubling the user's power level for the base Kaio-ken technique.
This short burst of power beyond the body's normal maximum output comes at the cost of increasing the flow of Ki within the user's body, taxing it and requiring great skill and concentration to keep under control. The uprise of ki flowing inside could easily obliterate them, and even when kept under proper control still causes intense muscle spasms, pain and weakness after prolonged use.
Whilst already a useful technique in its base form, the technique's true potential is in its ability for the power multiplier to be stacked, from doubling of the base Kaio-ken to a tripling, quadrupling and even further. The limits of the multiplier appear to be linked to the user's body toughness in addition to experience, so Goku is able to increase his durability and base strength through training and in turn gain an even higher power multiplier from the Kaio-ken. In Dragon Ball Z, when Goku first learned the Kaio-ken, the furthest he could handle was the Kaio-ken x4 (though his body was heavily damaged even after a x3 multiplier). However, through training at 100x gravity and then later with his dramatic strength increase after his fight with Captain Ginyu, he was able to withstand astounding multiplications of the Kaio-ken technique, such as Kaio-ken x10 and Kaio-ken x20 (which is said to be the maximum in the anime). The highest multiplication for the Kaio-ken ever seen was the Kaio-ken x100, which Goku used against Lord Slug.
In Dragon Ball Super, it was mentioned by Old Kai that using Kaio-ken alongside Super Saiyan is almost impossible or rather lethal because the intense emotional turmoil caused by the Super Saiyan transformation would not allow precise ki control to survive the technique. Only a fully controlled and calm Ki could be combined with the Kaio-ken.
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Kaio-Ken: Izuku will be able to use the Kaio-Ken technique and go as far as Kaio-Ken x10, and even x20 like he did once in his Past Life against Frieza, can only be stacked on his Base Form.
Maximum Kaio-Ken - A more powerful version of the Kaio-ken used by Goku as a Special Move in his God-Like Saiyan, Super Saiyan Blue, and GT Kid Goku (base form) in Dragon Ball Fusions. A single use puts the user in Kaio-ken+ status (the normal Kaio-ken requires the move to be used twice to achieve Kaio-ken+ status).
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Maximum Kaio-Ken: In this state Izuku's Hair will Spike Up and we will be using the Max Power of Kaio-Ken, can only be stacked on his Base Form.
Perfected Super Saiyan Blue Kaio-Ken: A combination of the Perfected state of SSBlue, with the Kaio-Ken technique, as it works perfectly fine, since he has to Heighten his Ki, without leaking it, making it much safer for Usage. (This will be called Super Kaio-Ken, as it still uses a Super Saiyan state) Multiplier is maxed at x10.
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(Credits to Dazaiel, since he came up with this Genius Idea)
Ultra Kaio-Ken: Since the Completed Ultra Instinct State, is Calm, has Calm Energy and has the needed Ki Control, it can be stacked with the Kaio-Ken Technique, making the Ultra Kaio-Ken possible, (Manga only, Anime MUI doesn't work with Kaio-Ken), the Max Multiplier will be a x5, but Izuku will be able to push himself to a x10, but it is Lethal and can Kill him almost instantly. Unlike the normal CUI state, Ultra Kaio-Ken has a Permanent Crimson Aura, since CUI only gets a Aura when they rush at the Enemy or fly at Max Power and Speed, the Aura normaly turns into the Giant Avatar State.
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Usages: ........ Kaio-Ken: not so often, because of the more Useful Super Saiyan States Maximum Kaio-Ken: read above Super Kaio-Ken: when needed, without wanting to go All Out Ultra Kaio-Ken: Only when absolutly neccessary
