"There's a form even beyond my current state right now. Let me show you a Super Saiyan's final stage, a Super Saiyan 5!" — Goku (Raising up the Playing Field! A New Power!)
Being the fifth and final form of Super Saiyan, it is the successor to all those that come before it. Goku was the first to achieve the form, through unknown methods. With Shenron's tremendous ki added on him, Goku was able to master the SSJ4 form, leading him to be a SSJ5.
The Super Saiyan 5 Form
The form is first achieved by Goku, whilst raining in the Dragon Realm with Pikkon. This form goes far beyond the hidden potential inside the user, as it to give rise to an immense surplus of power, speed, endurance. It is noted that Goku has transformed into this form many times unintentionally during AF, as Pikkon declares that he would stand a chance against Goku in battle if he didn't keep on changing into this form. However, in the battle with Xicor, Goku was finally pushed to put it into full use. In the confrontation with his evil son, Goku's SSJ4 transformation had failed him, leaving Xicor unscathed. In sure defeat, Goku slowly breaks a smile. He announces there exists a level that lives beyond that of a Super Saiyan 4. Xicor's grin fades in surprise, now intrigued for Goku's next move. Goku powers up similarly to when he powers up for Buu and Babidi, and the tremendous power shakes the ground and Xicor is blown away by it's force.
SSJ5 not only varies drastically from its predecessor in appearance, but also in power too. When SSJ4 Gohan, Vegeta, and Goku failed to harm to Xicor, the SSJ5 transformation completely blew away Xicor. Even Xicor thought to himself that Goku's newfound power surpassed his own.
This form possesses a major setback. The power density of this form is so great, the user is at risk of being collasped inward from its own power. All of their bones and organs will be crushed, slowly, until he condenses into a black hole the size of an atom, to small to do damage to his opponent, though will slowly consume the planet it lived on. In order to prevent this, the user must be cautious. If too much power is used, they will be actively fighting the inward force to prevent themselves from collapsing. As such, this form has only been summoned on only times of emergency.
Son Gohan had first used SSJ5 when he witnessed Pan's death at the hands of the evil tyrant who only wants to fight for pleasure, Ize. Gohan shook the Earth like Goku had, starting his vicious attack on Ize, who was absolutely surprised with the other Z Fighters.
Vegeta, in the fight with his father, saw his first transformation. His father, King Vegeta, was fighting him with six clones, firing the Ultimate Galick Gun. Then, while thinking about Goku, his inner power was tapped and transformed. Immediately, he absorbed his father's attacks, firing them as Ultimate Final Shine Attack. Afterwards, he found seven of his father's bodies, dead on the ground.
The transformation causes some stark changes in physical appearance. The trademark characteristic of the transformation is the user's hair: the hair of the Super Saiyan 4 state changes drastically, and grows down to or sometimes passes the user's waist, similarly like the SSJ3 form. But traces of the SSJ4 hair can be seen as well, forming bizarre hairstyle of silver hair. The brow ridge that is seen from the SSJ3 transformation is visible as well (though not as much as it is on the SSJ3). In this form, the Saiyan's hair color differs, the same goes for the tail and the body is covered in silver fur. A Saiyan in this form also possesses a shadow trim around the eyes and over the eyelids that varies in color, such as a dark bluish-gray or crimson. A similar aura like the SSJ4 transformation erupts in a SSJ5 as well. The combined looks of the two forms that come before this presumably marks this as the final form of a Super Saiyan.
In episode 55 of Dragon Ball GT, "The Heart of the Prince" , when Bulma tells Vegeta that she has been working on a way to get Vegeta to transform into a SSJ4, Bulma tells Vegeta that the use of her new machine may even make him go up to a SSJ5. This was probably only a joke, as the SSJ5 hasn't even been thought up yet and after Vegeta asks Bulma is she really believes the he can surpass Goku, she remains silence in embarrassment.
Though being depicted as a SSJ5, Vegeta became a super Saiyan 5 through fusion of porunga with Vegeta in Dragon Ball AF anime. During the battle, his father, who was battling in his super Saiyan form used multiplying technique, similar to Piccolo's. King Vegeta started firing the Ultimate Galick Gun when Vegeta suddenly, due to the enormous ki added by Porunga, transformed into a Super Saiyan 5. In his new form, Vegeta absorbs the attacks and fires them back as Ultimate Final Shine Attack. However, when he facing Xicor in combat, he only was a SSJ4 at the time, as well as being a SSJ4 against Ize and other antagonists occurring after that.
The Super Saiyan 5 Transformation is the only other form besides SSJ4 to be a hybrid between Saiyan and Great Ape, though it is unknown whether there is a Silver Great Ape in DBAF.
The major antagonist of the first saga of DBAF, Xicor/Zaiko, is said to resemble a SSJ5 in a few ways. One being its eyelids, another is the same crazy silver hair. In some translations of the manga, it is said that Xicor himself is a born SSJ5. Whether this is true or not, a fact that goes against this response is that the Western Supreme Kai/Lila (Xicor's mother) possesses a similar hairstyle with gray. He presumably inherited these traits from his mother, as he does not posses the fur or the tail that are require in this form.