The tyrant is the ego enraged when the steady faculties in a person refuse to bow to it. Nebuchadnezzar is the ruling self of the lower mind, and when it cannot bend the higher powers to its will it turns up the heat, stoking the inner fire to burn them out. The number 7 is the full spiritual measure, the complete set of centers in the body, so the furnace raised 7 times over is trial pushed to its total intensity.
Yet the fire meant to destroy is the very fire that refines. What the ego heats to consume the higher nature only tempers it and leaves it standing.
Nebuchadnezzar: the ruling ego of the lower mind, enraged when it cannot command the higher faculties
The furnace: the inner fire of trial and passion, the heat that either destroys or refines
7 times: the number 7, the full spiritual measure of the centers, trial raised to its complete intensity