Kader is the son of Emperor Valkorion and Empress Revan, he left the Eternal Empire to get away from the pressures of who he really wast — the Heir to the Eternal Throne.
As such, he ended up joining the Knights of Ren, and forsaking his official title for what he would call ‘freedom'
Kader is the son of Revan and Darth Xylek. Both of them are powerful in the Force and it was Xylek’s decision to let Kader do what he wanted — that meant joining the Knights of Ren.
Anything that is during the Clone Wars or before.
With Kader being forcefully on lock-down in Zakuul, he ended up contacting the First Order — specifically the Knights of Ren on how to gain access to Empire with out being caught.
That being said, Kader ended up creating another Arcann situation, and as such he became Emperor to the Eternal Empire after the murder of both his mother and Father.