From Starwars.com:
Thought dead, Darth Maul survived his injuries by focusing on his hatred of Obi-Wan Kenobi, the Jedi who cut him in half. His shattered body was dumped amid the refuse of the junk planet Lotho Minor, where the once deadly warrior fell into madness, staying alive on a diet of vermin. When found by his brother Savage Opress, Maul had no memory of who he was, rambled nonsensically, and had built hideous spider-like legs in place of his severed ones.
Opress brought Maul back to his homeworld of Dathomir, where the Nightsister Mother Talzin used dark magicks to restore the fallen Sith's mind and body. Maul reemerged, took Opress as his apprentice, and became obsessed with Kenobi. He and his brother began terrorizing worlds and allied with Death Watch, Black Sun, and the Pyke syndicate to form the Shadow Collective, a group which took over Mandalore. This led to Maul's battle with Death Watch leader Pre Vizsla, who he dispatched violently and without mercy to assume total control of the planet. Throughout this time, Maul dueled Obi-Wan Kenobi on numerous occasions, and in an act of pure hatred, killed Obi-Wan's former love, Duchess Satine Kryze. His revenge would prove double-edged, however, as the act created a dark side ripple sensed by his former Master, Darth Sidious. Sidious came to Mandalore where he declared the Zabrak brothers to be his rivals. The Sith Master killed Savage Opress and took Maul captive -- but not before joyfully torturing his old apprentice with Force lightning.