What is Brevia Magistralia?

Legal Definition
These were writs formed by the masters in chancery, pursuant to the stat. West. 2, c. 24. They vary according to the diversity of cases and complaints, of which, says the author of Fleta, some are personal, some real, some mixed, according as actions are diverse or various, because so many will be the forms of writs as there are kinds of actions. Fleta, lib. 2, c. 13, ยง4; Co. Lit. 73 b, 54 b.
-- Bouviers Law Dictionary
Legal Definition
Writs occasionally issued by the masters or clerks of chancery, the form of which was varied to suit the circumstances of each case. Bract fol. 413b.
-- Black's Law Dictionary