Each breakthrough to a major realm brought with it a tribulation, so long as one had a Dao.
From Saint to Immortal, Immortal to Divine, Divine to Celestial, and if there was a way beyond that, it would all have a tribulation that one had to go through to reach the next stage.
The tribulation that came when one went from Saint to Immortal was called an Immortal Tribulation but was generally referred to as Lightning Tribulation.
Instead, an Immortal Tribulation referred to something else entirely.
Because the world was slowly losing Qi over time, there was a rule created by the Sun God and Moon Goddess when they prepared the Heavens. Anyone under it who was an Immortal had to break through at least once every 10 thousand years. To test whether one had or not, every 10 thousand years, Heaven would test them with a tribulation that judged whether they deserved their Immortality or not.
That was what people understood to be the Immortal Tribulation now.
