The Stockwood Scrolls
Known as The Lord of the First due to her status as ruler of the Avernus (the first level of Baator), Zariel is an archdevil of the Nine Hells. It is believed that she is allied with Tiamat, who resides in Avernus.
According to Savenius, Savant of Waterdeep’s Font of Knowledge, Zariel has not been the ruler of Avernus for a long time, and in fact was formerly kept prisoner by the former ruler, Bel. For this he claimed the relationship between the two was antagonistic, but Bel nevertheless currently serves as one of her chief advisors.