Darth Vader's secret forces

In 0 ABY, the Sith Lord Darth Vader assembled a group of secret forces who would be loyal to him alone in order to investigate two matters: Vader sought to learn the identity of the Rebel pilot who destroyed the DS-1 Death Star Mobile Battle Station at the Battle of Yavin and the identity of an individual who was conspiring with Galactic Emperor Sheev Palpatine behind his back. For these matters, he hired the bounty hunters Boba Fett and Krrsantan, but he also believed he needed to assemble a private force of troops. For that, Vader recruited the rogue archaeologist Chelli Lona Aphra, who provided the assassin droids 0-0-0 and BT-1. Next, the group traveled to the planet Geonosis to retrieve a portable droid factory, which Vader used to construct his own army of BX-series droid commandos. After Krrsantan captured the individual who had met with Palpatine, Doctor Cylo, and 0-0-0 learned the scientist was working to create replacements for Vader, Vader led his commando droids into the skirmish in Cylo's research base.

After learning from Fett that the pilot who destroyed the Death Star was named Luke Skywalker, which Vader realized meant the boy was his own son, Vader made it his mission to capture the boy and turn him to the dark side of the Force. Vader, Aphra, 0-0-0, and BT-1 all attempted to capture Skywalker during the Battle of Vrogas Vas, but he escaped. Vader also deployed his commando droids into the War on Shu-Torun. When Palpatine learned the extent of Vader's secret army, he was pleased by how his Sith apprentice had acted in the shadows as it was the way of the Sith.

Text and facts from the “Darth Vader's secret forces” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.