The Dawning is a "Rite of Passage" that every Fae must undergo. It consists of several tests, and should a Fae fail to pass The Dawning they will devolve into an Under Fae. An acute sense of smell is the first sign of de-evolution.
Preparation for The Dawning is a grueling process which normally takes place when a Fae is 200 years-old. There are mentors and trainers, such as Stella Nashira, a Lodestar, who helps Fae prepare for their Dawning. Those who succeed at passing their Dawning are considered to be very strong (Tamsin mentioned to Acacia that she could not overwhelm Bo because she had already gone through The Dawning and passed it).
Although Lauren knows about The Dawning, humans are not meant to know of its existence, let alone its purpose, since it is as much a secret as the existence of the Fae among them.
The first step of the process involves stepping through a ritualistic doorway. In order to pass through the doorway, the Fae must resolve their inner conflicts. After passing through the doorway, the Fae will undergo a test. The test always varies in nature and the Fae may be unaware of when the test is occurring.
If the test is completed successfully, the Fae receives an official "Invitation" to the Dawning. The Invitation gives the Fae entrance to the final test, The Temple.
Because Bo was unknown to the Fae when she was growing up among humans, her going through The Dawning was considered unusual. Most Fae have two centuries in their lifetime to prepare for it, whereas Bo only had the time between her relatively-recent discovery by the Light Fae and the current period in her lifetime. Trick enlisted Stella's help to prepare Bo for it.
Unprepared for her Dawning, Bo was initially unable to walk through the Dawning Threshold because, as Stella explained, she had unresolved emotional issues. Bo realized she was unable to walk through the doorway because of her complicated relationship with her adoptive human mother, Mary. In There's Bo Place Like Home, Bo returned home to her mother and they were able to settle their grievances. This enabled Bo to pass through the Threshold.
Bo received her official "Invitation to The Dawning" in the form of a machine with levers, dials, and lens; however, due to unusual circumstances, Trick had to decide which lever to pull for each test presented by the Dawning Invitation Machine.
During the last test, Bo's experience inside Brazenwood was influenced by Trick and Stella when their hands touched the Machine during a passionate first kiss, causing it to spark, propelled Tamsin (who had accompanied Bo during each test) towards Bo, and compelled her to kiss Bo.
After killing her opponent in the last test, Bo saved a Squonk from a Fae drug lord. Having passed the tests of The Dawning, she was now ready to enter The Temple. Bo was told that she should take something as an aid along with her, and Dyson volunteered to be her "Hand" (Bo, however, did not know that only one person could return from The Temple).
Inside The Temple, Bo appeared to be in her normal, regular world – except that Lauren and Dyson had switched their personas. Bo found herself inside a police station with Lauren in police uniform sitting at the desk next to hers. Dyson appeared as a doctor in scrubs who was medicating Bo so that she would not remember her life as a Fae.
Just as Trick and Stella touching the Dawning Invitation Machine influenced Bo's experience in her last test, Dyson's feelings for Bo influenced the events within The Temple. This backfired as Dyson realized that Bo was beginning to devolve. He then reminded Bo that what was happening was not reality and that they were inside The Temple going through The Dawning. Bo finds out that only one of them could leave The Temple, and faced with obeying the rule which mandated that one of them had to stay, she defied the canon of The Temple and killed Dyson, taking his body with her as she exited it.
When Bo was back in the real world inside The Dal with Lauren, Kenzi, Trick, and Stella present, she tried to revive Dyson but was unable to, becoming increasingly distraught. Lauren performed CPR on him but was unsuccessful. Suddenly, Bo's persona changed as she became possessed by Dark Bo and chanted: "I will reign as he did, for I am his daughter. Together, we will bridle the masses and ride them to victory. Even death will fear us." Cognizant of what she was doing, she then drew the Chi from all of them (human and Fae), and transferred the energy into Dyson, bringing him back to life.