The Spirit Bomb is a powerful attack in Dragon Ball, widely considered to be potentially the strongest attack in the series, but it depends on the number of organisms supporting its use to determine it's strength.
History and usage
Users of the Spirit Bomb gather huge amounts of energy from all chosen surrounding life forms, and conduct that energy into a massive sphere of astounding destructive power. Energy takes the visual form of sparkling, glittering wisps when adding to the mass. The creation of the attack promotes a calm breeze away from the bomb, which turns into a strong continuous gale and expels colorful bands or radiation of Northern Lights-esque aurora. When used it is quite swift, and if the user is not careful of it, the Spirit Bomb could absolutely obliterate a planet.
Essentially, to use the Spirit Bomb, one must have a pure heart so they can manipulate and gather energy, otherwise the move can backfire and possibly hurt or kill the user. In the afterlife, Goku was taught this move from King Kai. Likewise, once a Spirit Bomb has been fired, it is possible for one with a pure heart to deflect the technique, as Goku clarifies to Gohan and Krillin. However, it should be noted that although whether a person can be pure good or pure evil of heart, a Spirit Bomb that is created by a pure good person can not be deflected by a pure evil person, the two opposite energies cancel each other out and one engulfs the other. This is shown when Frieza (a being of pure evil) is not able to deflect the Spirit Bomb, although being able to survive it through brute force. Therefore, the Spirit Bomb harms anyone, regardless of morality.
However, the main disadvantage of the attack is the amount of time needed to complete the attack, thus the attack is used very rarely. Although Goku, Krillin, Gohan, and Gokule are the only characters known to have used this attack, it was Goku who gathered the energy before Krillin threw it and Gohan only rebounded the attack, and in addition Gokule was half Goku. Cell also claims to be capable of using the attack (likely because he has data collected by Dr. Gero's insect tracking device about the fight between Goku and Vegeta).
Occasions it has been used
During the battle against Vegeta, Goku had no choice but to use the technique King Kai taught him: the Spirit Bomb. But Vegeta transformed into a Great Ape and due to the enormous disadvantage he was at against his now monstrous enemy, many of Goku's attempts failed. However, he eventually managed to collect the energy needed for his ultimate attack. Unfortunately, before he was able to unleash this attack on his foe, Vegeta thrashed Goku, reducing the amount of gathered energy and leaving him incapable of doing anything. Still, he managed to give what was left of the gathered energy of the Spirit Bomb to Krillin, and with some help from Gohan, the Spirit Bomb reached its target, Vegeta. Although a direct hit, the Spirit Bomb did not finish the Saiyan prince, presumably due to having lost too much power, although it did injure and weaken Vegeta.
Similarly to his fight with Vegeta, Goku couldn't defeat Frieza even with his Kaio-ken's power increased 20 times, so he decided to once again use the Spirit Bomb. With some help from Piccolo, Krillin and Gohan, Goku had enough time to collect energy from the remaining living beings of Namek as well as some nearby planets, thus creating a bigger Spirit Bomb than the last one. However, the Spirit Bomb was not strong enough to bring an end to the galactic tyrant. This is speculated to be either because Goku underestimated Frieza's power (since he was only at about 50%) or because there was very little life left in the Namekian planetary system. Despite the fact that it failed, Frieza was still unable to reflect it.
After seeing that Goku as a Super Saiyan 3 had not finished off Kid Buu to allow Vegeta a fair chance to fight, Vegeta came up with the idea to make a Spirit Bomb powered by the Earthlings. By restoring the Earth and immediately reviving everyone and the planet thanks to Porunga, every human would give Goku most of their energy. However, only Goku's friends gave their energy at first since everyone else had doubts about the voices they were hearing. But once Mr. Satan talked to the people on Earth and convinced them to raise their hands, everyone's energy became part of the Spirit Bomb, which eventually obliterated the evil Kid Buu (Goku formed the Spirit Bomb in base form, but had to go Super Saiyan as Kid Buu was deflecting it).
Having Goku and Vegeta lost both their fusion and their transformations as Super Saiyan 4s, the battle against Omega Shenron seemed practically lost. However, they would not give up and with a Spirit Bomb powered by the whole universe, the evil Shadow Dragon was beaten and completely destroyed.
During his attempt to fight against Maceza by himself, Gokule attempts to use the Spirit Bomb. Due to having been formed by Gokule instead of by Goku, the Ultra Spirit Bomb is pink instead of blue, and it is also much stronger due to being formed by a fused fighter. Like the Super Spirit Bomb, the Ultra Spirit Bomb uses the powers of the whole Earth, obtained by Gokule impersonating Hercule (as Hercule was one of his halves). It seriously injured Maceza, but it was not enough to destroy the monster.
- At first, Goku could only create basketball sized Spirit Bombs. Later on, he can create ones that are the size of miniature planets.
- Even though he is not pure of heart, Krillin was able to wield the Spirit Bomb when Goku gave it to him to attack Vegeta. That may be because a pure heart is only required to create the Spirit Bomb, not to launch it.
- Though neither character creates a Spirit Bomb, both Gohan and Kid Buu are contacted by the attack and manage to deflect it towards another target.