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.
If Maul had survived on Naboo, he probably would have been trained as a liason. He wouldn't lead the Seperatists the way Tyrannus did, but rather act as Sidious' messenger, enforcer, and spy. We see Maul approach this capacity after he arrives on Naboo.
Lightsaber inconsistencies aside, what kept Darth Maul alive initially was his hatred of Obi-Wan Kenobi. Revenge is a powerful motivator. Revenge plus a mastery over the dark side of the Force is enough to keep half a Sith alive long enough to rebuild themself.
Sidious revealed to Maul that he did not intend to kill his former apprentice, as he still had use for Maul; it was Sidious' hope that Maul would lead the Sith to Mother Talzin, Maul's mother and the leader of the Nightsisters, so she could be eliminated as a potential threat to the eventual Sith rule of the galaxy.
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How did Sidious react to Maul's death?
He regretted it because he had essentially wasted 20 years training him. But since he was an enemy at the time of his death on Tatooine, Sid was glad that he did not have to keep watching his back anymore now that the loose ends were tied off.
In a non-canon story from the pages of Star Wars Tales #9, Darth Maul was resurrected to fight Darth Vader to the death. Ultimately, Darth Vader was named the victor for a pitch-black reason: He hates himself more. In the main Star Wars canon, Darth Vader and Darth Maul never met, let alone fought.
Finally, Darth Maul himself admitted he wasn't capable of facing Vader alone in Star Wars Rebels. And while the story in question is no longer part of the official Star Wars canon, Darth Vader fought and defeated Maul in comic Star Wars Tales #9.
Maul's skills, on the other hand, became stagnant as he spent the last few decades of his life keeping himself alive and plotting schemes. Given this, it makes sense Obi-Wan quickly defeated Maul in their final duel in Star Wars Rebels.
Like his master, Darth Sidious, better known as Sheev Palpatine, Maul's main weakness was his overconfidence. In Luke Skywalker's confrontation with Palpatine during The Return of the Jedi, he tells the Emperor that his overconfidence is his weakness.
In this scenario, Maul would cause Luke Skywalker to fall to the dark side and then begin training him as an unofficial Sith. With Obi-Wan Kenobi dead, Maul's thirst for revenge wouldn't end, and he'd set his sights on a more dangerous opponent.
Mother Talzin, leader of a large sect of the Dathomirian Force Witches, in accordance with her own designs for revenge and conquest against Palpatine and his imperial allies, resurrected her son Maul using dark side Force magic to empower and repair his body and restore peace to his troubled mind so that he could focus ...
In Clone Wars, it's said that Maul refused to die thanks to his hate for Kenobi and for Sidious and that the Dark Side helped to keep him alive. In order to better explain Darth Maul's Clone Wars return, a fan theory (via Reddit) claims that he was either possessed by or had his soul combined with the Son of Mortis.
And while his warnings don't work, at the very least, it shows that Maul became aware of Anakin and what he would soon become. With the rise of the Empire, it's likely Maul caught on to Vader's existence pretty quickly.
By the time of Episode 3; Anakin is in his prime, and already one of the Jedi Order's most powerful “masters” (although technically still a knight), just behind Obi-Wan, Windu, and Yoda. Maul wouldn't stand a chance.
Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth. Instead, he was just a chaotic mess of the dark side (until he eventually found his way back to the light).
In 54 BBY, the boy who would be dubbed Darth Maul was born the son of Mother Talzin on Dathomir with mysterious origins. As a Dathomirian Zabrak male, he was a Nightbrother and had two blood brothers, Feral and Savage Opress.
Maul actually was his Sith name. In the Rebels tv series, Maul says to another character that he once had a real name, but he has long since forgotten it. Palpatine took Maul from his mother and gave him his Sith name as an infant. His real name is lost to time.
It's already been shown in the source material… more than once… that Darth Maul is not on Darth Sidious's level. Every time he dueled Sidious as his apprentice (in Legends) he got easily defeated.
Dooku despises Maul for being a threat to him, and for killing his beloved padawan Qui-Gon, and Maul hated Dooku for being his replacement. They also didn't regard each other as a true Sith: Dooku saw Maul as nothing more than a trained animal and Maul saw Dooku as a Sith pretender.
In the novelization he seems to feel… mild regret, maybe? He didn't expect Maul to die, and this might indicate a degree of respect for his apprentice and his skills; he didn't hold it against Maul that he lost, but honestly believed that the Jedi would be killed, so he might have felt that he underestimated them.
