In the wake of the X-Men’s sudden absence from the world (see below), the Mutant Underground is the one place those with the X-gene came feel safe from the threat of Sentinel Services. The MU is based in Atlanta, Georgia. The group is loosely based on the Morlocks from the comics, a group of victimized mutants who lived underground (literally, in the sewers).
Sentinel Services is a government agency committed to capturing mutant terrorists. Led by ruthless Agent Jace Turner, they employ deadly spider droids to track their targets. In Days of Future Past, the Sentinels were highly-adaptive giant humanoid robots that had driven mutantkind to near-extinction. They were created by…
Trask Industries’ Hound program is led by Roderick Campbell, who has devised a way to control mutants via a chemical drug. Campbell works closely with Sentinel Services. Again, in Days of Future Past, the company was run by Bolivar Trask (Peter Dinklage) who was determined to wipe out mutantkind.
Originally a wealthy mutant criminal group that existed from the 1940s to the 1960s, the Hellfire Club is being rebuilt by its mysterious Inner Circle in The Gifted. Several MU members defect to the group at the end of season 1. The Club was led by Sebastian Shaw – and then Magneto – in 2011’s X-Men: First Class.
The Purifiers are an anti-mutant hate group (similar to the real-world KKK) that commit violent attacks against mutants. In the comics, the movement is led by William Stryker (seen in many X-Men movies).
X-Men/ The Brotherhood
The X-Men and their rivals the Brotherhood of Mutants have both disappeared for unknown reasons in the timeline of The Gifted. It could perhaps be tied into the “7/15” mutant rights event that went south, leading to the deaths of thousands.
Or, if you want to fit the show into the timeline of the movies, it’s possible they have gone underground ahead of the apocalyptic events of Days or have even been killed by the ailing Professor X, as happened in Logan.