The word convertisteis is the preterite form of convertir in the second person plural.
See the full convertir conjugation

convertir

Lo convertisteis en un juego.
Turned it into a game.
Tropecé con vosotros y me convertisteis en un ídolo.
And then you boys came along. You looked up to me.
Sin embargo, ya os convertisteis en esto en el ciclo previo y ahora os tenéis que hacer así otra vez.
However, you did become like this in the previous cycle and you now have to become like this again.
Es por eso por lo que papá y tú os convertisteis en mis padres.
That's why Pa and you became my parents.
Os convertisteis en Io más importante de mi vida.
You became the center of my world.
Vos nunca habéis aceptado el hecho de que vuestra vida ha cambiado en el momento en el que os convertisteis en reina.
You've never accepted the fact that your life changed the moment you become queen.
El Padre tiene que venir por vuestro bien. Hijos, Él también os enseñó en el ciclo anterior, y después os convertisteis en los amos del mundo.
In the previous cycle too, He taught you children, and you then became the masters of the world.
No, lo siento, os convertisteis en toda mi família, y yo, realmente, no quería perder eso, y sé que dijiste que todo seguiría igual, pero...
No, I'm sorry. It's just, I-I—you've all become like family to me, and—and I really didn't want to lose that, and I know that you said that everything would stay the same, but—
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