Niang, 23, returns to France with Stade Rennais on a season-long loan deal from Serie A side Torino.
While his international teammate Kara, ended his five-year spell in the Belgian top-flight with a move to Nantes.
Like Niang, the robust central defender has also agreed a season-long loan from Anderlecht.
And the third high-profile arrival in the division is Nacer Chadli, joining AS Monaco from Championship outfit West Bromwich Albion for a reported £10m fee.
The 29-year-old impressed during Belgium's third-place run at Russia 2018 and has subsequently ended his five-year spell in England.