It was a national council[2] held against the Priscillianists (a schismatic sect with Gnostic-Manichaean, Sabellian, and Monophysite doctrine[3][4][5]), as called for by Pope Leo I.
[9] It gave a profession of faith against all heretics with 18 anathemas attached against the doctrines of Priscillian.
It was called for by Pope Leo I, in a letter preserved in the register, but there is no contemporary evidence that it was held.
The letter in question is Leo’s Epistula 15 (Migne, ed., PL 54, 678-695).
Vague mention is made of the supposed Toledo council at the First Council of Braga, held in 562, but some have argued that the bishops there were confused, and instead had a copy of the profession of faith from I Toledo.