When The Burst Brawlers battled Final Crush all of the brawlers guardians evolved into their most power forms. Demokro was last to transform as Final Crush still managed to take down all of the evolved guardian bakugan. Demokro unleashed his most powerful move and ability and with the power of it he evolved into Booster Demokro and unleashed the final blow.
When Cora discovered Krion and Wrath battling Sado,she and Booster Demokro tried to stop Krion. When they succeeded Krion challenged Wrath to a brawl and used Booster Demokro. They defeated Wrath and Fear but in the process actually destroyed them both.
When the Shredder Bakugan attacked The Burst Brawlers didn't stand a chance. Booster Demokro battled many but kept losing. In a fit of rage he unlocked the the true power of the Darkus attribute and evolved into Thunder Demokro.
You can see the energy radiating off him in his ball form.