According to BG:DIA, Zariel was an angel (a Solar, to be specific) who decided to disobey orders and attack the Nine Hells. To do this, she went to the city of Elturel in Toril in the year 1354 DR and gathered an army of mortals called the Hellriders. She then transported the group to Avernus, but the group was defeated and Zariel captured. She was taken to Asmodeus, offered ruleship of Avernus, accepted, and took over from Bel - presumably all in the year 1354 DR. The book goes on to tie Zariel to the Companion that hovers over Elturel, asserting that the Companion came about as a result of a pact between Zariel and Thavius Kraeg, which he made in order to get rid of a vampire king and his minions.
The adventure begins in 1492 DR, when payment for the pact comes due, the Companion turns dark and Elturel is drawn into Avernus. Over the course of the adventure, PCs can save the city and possibly redeem Zariel, resulting in her returning to her Solar form and somebody else assuming the rule of Avernus (possibly Bel, who is statted up as CR 25, but this is left open.) Now, assuming she is either redeemed or killed, her rule would go from 1354 DR to 1492 DR, or a total of 138 years by my count - a remarkably short period in my opinion. As far as this adventure is concerned, Zariel ruled Avernus for one period, and was never Bel's prisoner, contrary to previous lore, where Zariel ruled first, then Bel took over, then Zariel resumed her rule.
That's not to say the entire book is garbage - it's not, there's some good material here, including stats for Kotchtchie (also CR 25), some fun war machines, and the Infernal script at the back of the book - but yeah, it pretty much throws canon to the wind.