Savage Opress is a Dathomirian Zabrak Nightbrother, brother to Maul and to Feral, introduced in "Monster" — the thirteenth episode of the third season of Star Wars: The Clone Wars, first broadcast on 14 January 2011. He was voiced by Clancy Brown.
Made, not recruited
Asajj Ventress selects him from the Nightbrother village as a candidate weapon against Count Dooku, and Mother Talzin physically remakes him with Nightsister magick — larger, stronger, and stripped of the judgement that would let him refuse. The transformation scene is deliberately grotesque, and the series never lets you forget that what is standing there afterwards is a person who was altered without consent.
The cost, stated immediately
His first act after the ritual is to kill his brother Feral on Ventress's order. The episode makes the point in the bluntest possible terms: the thing they have built cannot tell the difference between an enemy and a family member, which is what makes it useful and what makes it worthless.
What he is for
Narratively he exists to bring Maul back. Talzin sets him to find his lost brother, he finds a broken half-machine thing on a junk world, and the two of them go on to take Mandalore. Savage is the bridge between Maul's death in 1999 and Maul's return as a functioning character, and the show is honest that this is his role — he is never the protagonist of his own story.
Canon and Legends
The Clone Wars is canon, and Savage is canon. The Nightbrothers and Nightsisters as institutions originate on the Legends shelf, in Dave Wolverton's The Courtship of Princess Leia (1994), where the arrangement between the two groups is described quite differently. Where the shelves disagree, this dossier follows the series.