The word convertíais is the imperfect form of convertir in the second person plural.
See the full convertir conjugation

convertir

Todos os convertíais en piedra.
All of you turned to stone.
Want to Learn Spanish?
Spanish learning for everyone. For free.
SpanishDict Premium
Have you tried it yet? Here's what's included:
Cheat sheets
No ads
Learn offline on iOS
Fun phrasebooks
Learn Spanish faster
Support SpanishDict