Legendary Super Saiyan is a unique transformation that is only reached by one Saiyan every thousand years
Legendary Super Saiyan (Dendetsu no Sūpā Saiya-jin) is a unique and very rare transformation rather than just a title that is reached by one Saiyan every thousand years however it is not a manner of training to reach the form, the Saiyan is genetically chosen to be a Legendary Super Saiyan, Broly is one of the Legendary Super Saiyans, the Original Super Saiyan was the first Super Saiyan to exist which positions him as the first Legendary Super Saiyan and it has been stated that his power was immense and tremendous that he was only able to control his power in his Oozaru form and it is likely that he resembled the Golden Great Ape however he eventually lost control of his power and destroyed the Saiyan's original home planet along with himself. (Only one Saiyan is said to be born with this power once every thousand years or so.) In Broly: The Legendary Super Saiyan, the audience is introduced to Broly, who is (as the title suggests) the Legendary Super Saiyan, a warrior that tales tell is truly the strongest fighting being in history. Broly's final transformation puts him in a class of his own. The result is a Super Saiyan with a seemingly endless supply of ki.
Beyond his initial Super Saiyan transformation, Broly's final form is very similar to a 3rd Stage Super Saiyan, but without the same physical flaws of massive chi consumption and speed loss. This form has many advantages when compared to the other Super Saiyan forms, as it requires no chi consumption, gives the user phenomenal strength and speed, and constantly raises the user's power as long as they remain in the Legendary Super Saiyan form. In this form Broly's size, muscle mass and power therein are far beyond that of any Super Saiyan (the ground quakes beneath his feet as he walks). His aura becomes electric green in color, and is so forceful that it can displace solid matter. During the initial transformation, there were bio-electrical discharges comparable to lightning bolts due to the sudden massive increase in energy flow and steady increase of power, which will overwhelm the user if it is allowed to continue, but these dissipated when Broly's power stabilized by releasing a large wave of energy to burn up the excess in order to prevent his energy from overflowing. Additionally, his irises and pupils seemingly disappear (metaphorically this is called "blind rage").
His transformation magnifies the power of all of his attacks, without drawbacks. In fact, Broly was so overwhelming, that he was able to take down two Full-Power Super Saiyans, two 2nd Stage Super Saiyans, and a Super Namekian. He also battles Super Saiyan 2 Gohan in Broly: Second Coming and completely dominates, though to Gohan's credit, Gohan's blows managed to cause Broly some injury, enough for him to be able to escape Broly's grip.
It is also stated in the OVA, Plan to Eradicate the Saiyans that a Legendary Super Saiyan destroyed the Saiyans' original home planet (this one being the Original Super Saiyan).
As already stated, Broly's appearance in this form is, at first glance, not much unlike that of a Super Saiyan 1 Stage 3. However, there are several notable physical differences. For example, Broly's muscle mass becomes greater than that of a 3rd Stage Super Saiyan. In order to maintain this increment of muscle mass proportionate to the body, Broly's height also increases substantially. When in this form, Broly's pupils and irises become seemingly invisible, making him look much more maniacal than he already is. They stay this way as long as Broly remains in this form. Broly's hair in this form is very wild and glows green in color; it would later be a natural, golden-yellow when the effects of a control device on his head wore off.
In this form, Broly grows to enormous size. His power, speed, strength, energy, and endurance increase drastically, and continue to do so steadily as long as he remains in the form.
This form is also one that is quite unique to Broly (as it is a genetic trait), as he is the Legendary Super Saiyan. It is a form that has very little disadvantages and allows Broly to overwhelm his opponents. Broly's power level not only shoots upward tremendously, but it also steadily increases as long as he stays in this form, so the longer he fights the stronger he gets. Since the power level in this form is so vast, Broly has been shown to take extremely powerful attacks without so much as flinching. For example, in his first appearance, Broly fought two Full-Power Super Saiyans, 2 2nd Stage Super Saiyans, and a Super Namekian all at the same time and was in control of the whole fight, with his opponents being unable to hurt him in the least. Even a Kamehameha from Goku at point blank range didn't do anything to Broly. It was only after everyone donated their powers to Goku that Goku was able to surpass Broly in power and put him out of commission temporarily. In his second appearance, Broly goes head to head with Super Saiyan 2 Gohan. While Gohan is able to damage Broly slightly more so than anybody from the first appearance was, Gohan still finds himself to be overwhelmed greatly, and Broly is only taken care of with some help from Goku, Goten, and Trunks.
Broly's attacks are also unique. In terms of physical attacks, he prefers to use wrestling moves to overpower his opponents rather than using lots of fast and powerful punches and kicks like most other fighters in the series use. He often uses clotheslines, crushing moves, grabs, and other such moves (he can also perform fast attacks, but prefers the physical moves above instead). His energy attacks are also one of a kind. They are all a light green shade in color and there are three main types of attacks that he uses. One type is a small, single ball of energy, which he controls much like somebody would control a toy remote helicopter or airplane. He uses this to make it a lot more difficult for his opponents to get away from his attacks. Another type is a large number of these blasts all thrown at one general target at the same time. Broly uses this attack to blow up cities, as well as to aim at enemies that he wouldn't be able to hit very well with the single ball of energy. His third, and most dangerous and powerful, energy attack, is the one that starts off as a compact sphere of energy, but upon hitting its target, it turns into a enormous sphere of energy with immense destructive power. Broly has used this attack to destroy planets as well as to counter attacks.
Broly also appears to survive exposure to magma in this form, though this trait is common with the other Super Saiyan forms, as Goku also survived lava exposure during his fight against Frieza without any noticeable damage.