Henri III of France was in many ways a tragic figure. Despite having spent much of his younger years pushing for a harsh stance against protestantism he quickly became more and more moderate after his corronation, granting limiting toleration to the Hugenots with the Edit of Beaulieu in 1576 and refused the succession rights of his cousin and brother-in law, Henri of Navara (the OTL Henri IV).
Those policies lead to many catholic extremists to turn against the king, eventually rising in revolt against him and expulsing him of Paris, threatening to depose him. The fortunes of the king recovered, however, thanks to an alliance with the Hugenots. In 1589 he seemed poised to launch a successfull assault of Paris but was assasinated by the monk Jacques Clément, after witch many of his catholic supporters defected as they refused to serve his protestant successor.
Only Henri IV military and political talent, as well as a perfectly timed conversion allowed the royalists to recover and reunify France on their own terms.
What if Henri III had survived? Would he have, indeed, taken Paris soon afterward? How would he fare against the Catholic League afterward? What would his foreign policy be? Would he manage to produce an heir latter on? Wheter he does or does not, what would be the faith of the OTL Henri IV ITTL? What kind of religious settlement will he push forward if he is ever in a position to do so?