Emperor Realm level nine, with each level one has to go through a tribulation, in order: Earth-Slaying Disaster, Heaven Gang Tribulation, Heart-devouring Demon Tribulation, Wind Fire Thunder Tribulation, Five Elements Tribulation, Great Dao Tribulation, Myriad Forms Tribulation, Broken Tribulation, and Nirvana Tribulation.
Generally speaking, for the Earth-Slaying Disaster, Heaven Gang Tribulation, and Heart-devouring Demon Tribulation, as long as one doesn't have too poor talent and is prepared, they can pass through safely without too much trouble.
However, starting from Wind Fire Thunder Tribulation, each tribulation is like an insurmountable gap. Therefore, the Wind Fire Thunder Tribulation is considered a major watershed. In the Emperor Domain, only after passing this tribulation can one barely enter the list of the strong.