Darth Maul is a Dathomirian Zabrak, Palpatine's first apprentice, introduced in Star Wars: Episode I The Phantom Menace (1999) and performed by Ray Park with the voice of Peter Serafinowicz. He speaks a handful of lines, kills Qui-Gon Jinn, and is cut in half — which should have been the end of it.
The most rehabilitated character in the franchise
Instead he became a case study in how ancillary fiction can rescue a thin idea. The Clone Wars brought him back with mechanical legs, a brother and a Mandalorian crime empire; Solo put him at the head of Crimson Dawn; Rebels gave him an ending. Almost none of what makes Maul interesting is in the film he was invented for.
In print
The books have carried a lot of this. Michael Reaves' Darth Maul: Shadow Hunter (Del Rey, 2001) is the well-regarded Legends novel of his final assignment before Naboo; Joe Schreiber's Maul: Lockdown (2014) is a prison novel with a Sith apprentice in it, and it is exactly as strange as that sounds. Current canon's Maul material is spread across comics and the animated series rather than a single defining novel.
Canon and Legends
Both continuities have him; the Dathomirian and Nightsister origin, and the survival after Naboo, belong to current canon. Legends killed him and then resurrected him separately in its own comics, with different mechanics and a different aftermath. Keep the two apart.
Reading note
Maul is the strongest argument on this site for reading outside the films. On screen in 1999 he is a design. Everything that makes him a character was added later, mostly by other people, mostly in print and animation.
