The Lost Twenty was a group of twenty Jedi Masters who renounced their allegiance to the Jedi Order at various times in its history. Every member of the Lost Twenty had forsaken the Order after becoming disillusioned about its principles, indeed the Order’s very way of life, according to Jedi Master Yula Braylon, who served on the Jedi High Council.
Jedi Master Trennis left the Order in what Grand Master Yoda recalled to be a sad event. During the late era of the Galactic Republic, Jedi Master Dooku turned his back on the Order. He left before the death of his former Padawan, Qui-Gon Jinn, at the Battle of Naboo in 32 BBY. At some point Dooku embraced the dark side, and he adopted the name of Darth Tyranus upon becoming the Sith apprentice of Darth Sidious—the reigning Dark Lord of the Sith during the final years of the Republic. Publicly, he retained his former identity while serving as the face of a secessionist movement, and reclaimed his birthright as the Count of Serenno.
By 22 BBY, in the final days leading to the Clone Wars, the group was commemorated by the Jedi Order through a series of sculptural portrait heads, one for each member of the Lost Twenty. The busts were located in the Archives of the Jedi Temple on Coruscant.