In the play, it’s revealed that Dr Brenner’s dad was part of the Philadelphia Experiment, with USS Eldridge briefly traveling to Dimension X (aka the Abyss).
Dr Brenner is obsessed with opening a new portal to the Abyss. Prior to the Hawkins Lab experiment, he had an experiment set up in that desert cave. Henry stumbled across it, ended up in the Abyss, and got genetically mutated by the mind flayer.
The mind flayer also got into his brain, slowly turning him more and more evil. Dr Brenner did some tests on him, and it was the mind flayer who convinced him to murder his mother because she was threatening to hand him over to Dr Brenner.
At some point, Dr Brenner confronted him on stage at the school during Joyce’s play – I don’t recall whether it was during a performance or a practice.