I got this from a Quora answer:
1)When Taimur won war with Ottoman emperor Bayezid, he had emperor put in a cage like animal. He also made empress of Bayezid serve wine naked in his court. There was so much psychological trauma on Ottoman royal family that after that event no Ottoman emperor married so as to avoid shame of humiliation of empress in case such situation occurs again. Ottoman emperors just had concubines but no empress thereafter.
Note: Having concubines seems to be against Islamic teaching of not having sex outside marriage.
Wikipedia takes a differing view (without clear references to support it):
Many writers claim that Bayezid was mistreated by the Timurids. However, writers and historians from Timur's own court reported that Bayezid was treated well, and that Timur even mourned his death.