Legendary Super Saiyan Sonic is a form created by Supershadow124 and Marnicx in their Flash series DBZ Chaos. Most of the details of this form from that series do not exist because I haven't seen this series. However, some of the details from other fanfics and such will appear here.
Sonic uses this form in his fight against Insane Hell Pyrosis Shadow, losing badly because of a lack of Rings and Shadow's skill with that form. After being forced to power down, Sonic reabsorbs the energy from the form, and transforms to Darkspine Sonic 9, but is then fused with Shadow and Silver the Hedgehog who was unconscious to form Phoenix.
Random Fight/Fusion Saga
After HyperBlazing Will beats Phoenix and MegaKurai Mephiles and makes Phoenix defuse, Sonic attempts to transform into all of his forms at once, resulting in this form. Will is stunned by it's power, and lets Sonic escape. He later finds Mephiles who had escaped, obliterating him with a beam attack, before being attacked by his Heartless self and Heartless Shadow. Heartless Sonic fled while Heartless Shadow fought Sonic, losing badly, especially when he was double-teamed with BloodSonic and LSSJ Sonic. Later, Dark Sonic and Demon Shadow escaped the Sol Emeralds, and destroyed Chrome City. Sonic and Shadow, both in Legendary Super Saiyan forms arrive and start to own their dark counterparts. Sonic is later seen hurling Demon Subspace Sonic at the ground before powering down. The next time Sonic transforms to this form was a few months later when he was separated into his Super, Darkspine and Demon forms. Topaz, supposedly Sonic's father, escaped Sonic's body, and teamed up with Demon Sonic to fight Super and Darkspine. Both of them started to lose, so Super ascended to LSSJ and started to beat Demon (who had transformed higher as well), starting to lose when the Chaos Emeralds were scattered across Mobius by The Trolls. Super later returns to this form after absorbing Dark Sonic, Demon Sonic and the negative energy from Darkspine before that negativity transformed him to Corrupted Legendary Super Saiyan Sonic.
None as of yet...
Legendary Super Saiyan Sonic is near-invincible. While a Super form can fall through a lot of damage, this form barely takes damage from any other transformation, and in the times that he has transformed to this, it was only luck or outside events that forced him to lose. He is also immune to Chaos Control, able to reverse the time-slow or time-pause effect of it. It has all the powers of every other form Sonic has unlocked before it, and then amplifies that power to near-infinite.
- This is a DBZ form on a Sonic character, and of such has escaped all purges.